adds support for manipulating UIDatePickers with LPDatePickerOperation

* calabash_cucumber/date_picker.rb
    * contains date picker related functions
    * adds picker_set_date_time(<DateTime>) function for interacting
      with LPDatePickerOperation

* step_definitions/calabash_steps.rb - added 3 predefined steps
    * Then I change the date picker time to "10:45"
    * Then I change the date picker date to "July 28 2009"
    * Then I change the date picker date to "July 28" at "15:23"
  to serve as examples of how to use the new operation

* et.al. made the changes necessary to add the new modules to the gem

KNOWN ISSUES: does not support pickers in count-down mode

NOTE:

* operations.rb removed an ancient commented out function
  change_slider_value_to that came from briar because:
    a. it was commented out
    b. it did not work very well