Add tests around default section removal

There currently isn't any test coverage around SSSectionedDataSource, so
I wanted to make sure there were some minimal assertions in an area
before I addressed what I believe to be a bug.

Here we're just asserting that when you remove the last item or set of
items from a section, the section itself is also removed from the data
source.
Implement proper removal of section items

Before this commit, if you were to remove all of the items from a table
view section, SSSectionedDataSource would remove the SSSection (as
expected for UI concerns), but it did not actually remove those items
from the section.
Refactor implementation of -removeItemsInRange:inSection:

It can just delegate to -removeItemsAtIndexes:inSection:
Merge pull request #17 from andrewsardone/section-items-removal

Proper removal of remaining SSSection items