Adding filter support and tests for string, date_range, numeric, select, and check_boxes filters.
Merge pull request #50 from potatosalad/filter-feature

Added filter support and tests for string, date_range, numeric, select, and check_boxes filters.
Remove meta_search from deps

It conflicts with Rails 4 dependencies and is already required by activeadmin 0.6.

Credit to @iAmeen for this change.