Add optional Chosen integration

For multiple filter selections
Add check for filterType multiselect

For use with Chosen integration/multiple filter apply
Check for Select2 as well

Since Chosen and Select2 APIs are nearly identical this will allow developer to drop in either.
Merge pull request #14 from alexgann/chosen

Chosen/Select2 Integration
custom sortFunction in columnLabes. Issue #17
Refactor date/time specs.
Merge branch 'master' of github.com:rjackson/pivot.js
Fix issue with Safari not parsing millis since epoch properly.
Merge branch 'master' of github.com:m4urer/pivot.js into m4urer-master
Add tests for using a sortFunction with columnLabelable fields.
Simplify usage of sortFunction.
Add documentation about sortFunction and displayFunction for fields.
Update docs.
Update Gemfile.
Update demo CSV data.
Merge branch 'master' into gh-pages