Datepicker extracted to event. Jquery-ui-rails. Callback docs.

I redid this fork/pull request since the history was pretty messy. I had a lot of trouble getting git rebase to work. Now if you merge this the commit history won't be messed up.

Changes:
* Datepicker code has been extracted into an event that can be overridden for customization. An override for Bootstrap datepicker is included.
* Callbacks are listed in the README.md, and now all are tested.
* Code changed to reflect that jquery-ui is no longer included in jquery-rails. 
* Misc javascript improvements (semicolons, etc.)

All tests are passing in Rails 3.