Add nornsctl_iotask_init()
Add NORNSCTL_IOTASK()
Add nornsctl_submit()
Add nornsctl_wait()
Add nornsctl_cancel()
Add nornsctl_error()

We need to provide a nornsctl_status() function to check the status of
administrative data transfer tasks, but we already have
a nornsctl_status() function in the API that checks the global status of
the service. Thus, we decided to keep the current nornsctl_status() and
rename norns_status() to norns_error() and provide a nornsctl_error()
for simmetry.

Resolve "Add support for remote transfers"

Closes #21 and #24

See merge request !8

Transfer plugins now receive a new 'context' object that provides access
to relevant data structures from the daemon. This way, they can query
information about the current configuration and status without directly
involving the daemon object.