Proper removal of remaining SSSection items

I'm going to assert that there was a bug in `SSSectionedDataSource` with respect to removing items.

When removing all items from a section (e.g., via `-removeItemAtIndexPath:`), `SSSectionDataSource` [removes the empty `SSSection`][removal], but it doesn't actually remove the items. This results in an `SSSection` that has been removed because it's empty, yet it's really not empty.

I think the items should be [removed from the section][removal2], and *then* the section should be [removed if it's empty][fix].

[removal]: https://github.com/splinesoft/SSDataSources/blob/6d88b6c1eb3bbaefe6fa711bf541ca6ed04e16e9/SSDataSources/SSSectionedDataSource.m#L258
[removal2]: https://github.com/splinesoft/SSDataSources/blob/6d88b6c1eb3bbaefe6fa711bf541ca6ed04e16e9/SSDataSources/SSSectionedDataSource.m#L262
[fix]: https://github.com/andrewsardone/SSDataSources/commit/38aa7a8a57164d6caf4a68b9b4b7bb6c61ff3e55