Further improvements to timeseries filtering.

Updated ```qiime.filter.sample_ids_from_category_state_coverage``` to address the comments [here](https://github.com/qiime/qiime/pull/459#issuecomment-10896950).

Changes include:
* The set of covered states is returned instead of the number of covered states.
* A new (optional) parameter ```splitter_category``` allows splitting of the sample IDs into distinct groups that are each filtered separately (and have separate results returned for each partition).

This resolves issue #453.