Merge configure_sources changes/fixes from Michael Nelson
This branch fixes two issues I came across while working on a charm:
1) fetch.configure_sources already accepted either a list of sources with list of keys (where individual keys could be None), but not a single source with a None key.
2) fetch.add_source() didn't pass sources starting with 'deb ' through, and apt-add-repository gives a different result depending on whether the source starts with 'deb ' in some situations (see example in the test doc string).