Combine source_created and source_changed

As discussed in #214, source_created and source_changed didn't really
have clear definitions. In truth, their names and separation betray
their origins as model receivers in earlier versions. The "source group"
abstraction helped us get away from thinking about things exclusively in
terms of models, but these remained as an artifact.
Separate source group tests; signal counting util
Make test less verbose
Test that source_saved is dispatched for new instance
Add failing test for source_save signal w/o source

This is like the test contributed by @saulshanabrook in #214, but
catches the bug closer to its source. (A more unit-y unit test.)
Don't send source_saved when no source

Fixes #214 and the failing test from 404fed5
Remove source_deleted signal

...for now. Eventually, we will want a signal that tells us when sources
are no longer used, however that isn't just limited to when they're
deleted! This new signal should also be dispatched, for example, when
a source image field is set to `None`. Since none of the built-in
strategies are currently using the source_deleted signal, I've decided
to remove it until we have a more complete solution.
Merge branch 'source-signal-cleanup' into develop