Better timepicker support

I'm glad you added a datetime_picker_input method, but I would also like to be able to:

a) Pass in a time as a :value to datetime_picker_input and have it be parsed as a time, not a date

b) Pass in options such as :hour, :minute, etc that are defined in http://trentrichardson.com/examples/timepicker/

Both of these should be easy fixes to form_helper.rb .
