Fixed small bugs that were previously undiscovered during the MS handler
refactoring.  Namely, ensuring that coordination changes occur
correctly, Gateway deletions occur correctly, and API methods requiring
the calling user to be passed as an argument correctly get it (in
storage.py).

This commit also adds a small security fix:  the RG will ensure that a
block or manifest POST'ed came from a UG with CAP_WRITE_DATA, and a
block or manifest DELETE request came from a UG with CAP_COORDINATE.
This commit binds the appropriate methods into the Python API needed for
this to happen.