Added utility method for opening a file either in vospace or locally.
Astrom parser can now parse data directly from vospace if passed a 
vospace URI.
Changed open_vos_or_local to not be a context manager.  As seen in the
changes to the WorkUnitBuilders, we often want to leave the file open
and close it ourselves later.
Moved DirectoryContext class to a new context module and renamed 
it to LocalDirectoryContext.
Extracted usage of os.path.exists in persistence module to a new
exists method on the LocalDirectoryWorkingContext class (renamed 
from LocalDirectoryContext).
Extracted usages of the os module from persistence into the 
WorkingContext interface.  This should make it easier to switch over to 
VOSpace by providing a VOSpace counterpart to the 
LocalDirectoryWorkingContext class.
Extracted WorkingContext abstract class, added quick implementations
of VOSpaceWorkingContext.  Added integration tests for
VOSpaceWorkingContext.  Added factory method for creating the
appropriate WorkingContext class.
Merge branch 'master' into vospace_working_dir
WIP: trying to get append to work in vospace
Merge branch 'master' into vospace_working_dir

Conflicts:
	src/ossos-pipeline/ossos/astrom.py
	src/ossos-pipeline/ossos/storage.py
Merge branch 'master' into vospace_working_dir

Conflicts:
	src/ossos-pipeline/ossos/gui/app.py
Renamed ProgressManager to LocalProgressManager.
Added more generic methods for getting and setting properties
in VOSpace.
Changed get_property to return None if the node doesn't have the 
property.
Implemented progress manager interface using VOSpace tags.  
Testing still to be done, so this should be considered more of a sketch of 
how things will work.
WIP: setting up vospace integration tests for the vospace progress 
manager.
Implemented VOSpaceProgressManager integration tests that test the 
is_done, record_done and get_done methods.  Fixed bug in get_done.
Added unit_test and integration_test targets in Makefile.
Added additional VOSpaceProgressManager integration tests.  Fixed 
bug in record_index.
Added SyncingVOFile class which manages reading and writing files in 
VOSpace while minimizing the network traffic and maintaining a local 
copy in the temp directory.