- Add transferor_registry class to manage resource transfer functions
- First implementation of 'local path' to 'local path' transferor using
  fadvice+fallocate+sendfile.
- Add stubs for other converters
- Move dispatch_table from api to common since it's now also used by
  transferor_registry.
- Add get() function to dispatch_table class so that stored functors
  can be retrieved.
- Generic implementation of task::operator()() to transfer resources and
  resource elements between different backends. When constructed, the
  task is provided with the input and ouput resources as well as a
  'transferor' functor that is invoked upon them to perform the actual
  transfer.
- Add new_resource() and get_resource() methods to backends to promote
  resource_info instances to fully qualified resources.
- Implementation of new_resource() and get_resource() for a POSIX
  filesystem. Stubs for other backends.
- New convenience library that aggregates all symbols for resources
- Add name() and is_collection() methods to class resource_info.
- Provide implementation for boost::filesystem::relative() in utils.cpp
  if boost version <= 1.6.0
- Remove resource::buffer, no longer needed (superseeded by transferors).
- Remove resource::stream, no longer needed (superseeded by transferors).
- Remove functions resource::info(), resource::backend(), and
  resource::set_backend().
- Reorganize resources build hierarchy
- Remove self-registration of backends
- Update tests
- Cleanup unused code

- Rename norns_register_backend to norns_register_namespace
- Rename norns_unregister_backend to norns_unregister_namespace
- Rename norns_update_backend to norns_update_namespace
- API is now more symmetrical: all norns_*_namespace functions
receive an nsid as first parameter (in previous versions this
was not consistent).
- The norns_job_t descriptor now requires a norns_job_limit_t
parameter that defines what the job will be allowed to do, rather
than a norns_backend_t specification as in previous versions.
- Other changes: message passing logic improved

- task_manager is now a full-fledged class rather than
inheriting from std::unordered_map
- changes to handlers to reflect the new interface
in task_manager
- new dry_run option for enabling "fake task mode"
- in libraries, log errors to stderr only if env var
NORNS_DBG_LOG_TO_STDERR is defined.

Added support for the API listener to receive requests from
different endpoints: local sockets for user/control requests and
remote sockets for requests from other daemons. Signals are also
managed by the API listener.

Create common.hpp to access them

Rename handlers:

urd::create_task      -> urd::iotask_create_handler
urd::check_on_task    -> urd::iotask_status_handler
urd::ping_request     -> urd::ping_handler
urd::register_job     -> urd::job_register_handler
urd::update_job       -> urd::job_update_handler
urd::remove_job       -> urd::job_remove_handler
urd::add_process      -> urd::process_add_handler
urd::remove_process   -> urd::process_remove_handler
urd::register_backend -> urd::backend_register_handler
urd::update_backend   -> urd::backend_update_handler
urd::remove_backend   -> urd::backend_remove_handler
urd::unknown_request  -> urd::unknown_request_handler

Also, replace all static_cast<T*>(base.get()) constructs by
utils::static_unique_ptr_cast<T>(std::move(base)).
For example:
    auto request =
        static_cast<api::iotask_create_request*>(base_request.get());
becomes:
    auto request =
        utils::static_unique_ptr_cast<api::iotask_create_request>(
            std::move(base_request));

Initial implementation of a basic I/O task

Better abstraction and internal management of data
resources and storage backends

Bump spdlog to 0.13.0
Bump catch to 2.0