Significant changes to data resource management

- 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