Added theme capability

Added option 'datatheme' to allow choosing theme swatch in jQuery
Mobile.  This option defaults to 'a'

Closes #14
Closes #16
Custom callbacks allowed

Perform a customization of callbacks onSelect / onChangeMonthYear /
beforeShow without loosing mobile style.

Fixes #19
Closes #18
Closes #8
Theme is missing from options declaration

Fixes #22
Updating demo link in readme 

Fixes #15
Merge remote-tracking branch 'upstream/master' into second

Conflicts:
	jquery.mobile.datepicker.js