Added filter support and tests for string, date_range, numeric, select, and check_boxes filters.

The title describes the majority of this pull request.

I used all of the terms from [MetaSearch::DEFAULT_WHERES](https://github.com/ernie/meta_search/blob/master/lib/meta_search.rb#L12-L33) and added the corresponding Mongoid queries to get the filters to work.

I added tests for string, date_range, numeric, select, and check_boxes filters as part of the smoke spec.  I also added [simplecov](https://github.com/colszowka/simplecov) so I could gauge my testing progress.  It's disabled by default, but can be enabled with:

```bash
COVERAGE=1 bundle exec rake
```