Rails 4.1 support

I started with @hale's fix, upgraded rails to 4.1.6, and made a handful of changes to get all the tests passing again. The switch from InstanceTag to Tags::TextField implies this will only work for Rails 4.1 and later.

I also re-factored it a bit (separate commit) to avoid needing to copy parts of the options hash between a JqueryDatePicker::InstanceTag and a TextField. Now you can directly render the JqueryDatePicker::InstanceTag. All tests should still be passing.