Try calling send_extended() with address argument as well
Clean up some comments
Updated for the fact that Marquise handles now have two spool files

Metadata, in the form of source_dicts, are appended to a separate spool
file. As such, we now need to define two separate spool files.
Implement update_source()

This gives users the ability to submit a dictionary of arbitrary
key-value metadata pairs to assign to an address.
Add some test calls to the new update_source() function