Fixes scroll issue in position:fixed elements

If the datepicker is placed within position:fixed elements, e.g. modals and fixed navbars,
scrolling the page also moves the datepicker away.

Tested when placed within:
- normal element, e.g. within a form
- fixed element
- modal

Tested on:
Chrome 25 (MacOS 10.8)
Safari 6 (MacOS 10.8)
Firefox 19 (MacOS 10.8)
Internet Explorer 7 (Windows XP)
Build minified script and updated contribute guide
Added an option to default a date's time component as 12am. Option is set with an optional property, setCurrentTime {Boolean}.
Overlooked setting a date when clicking a month from the month picker. Also overlooked toggling AM/PM.
Change to allow the datetime picker to update when a date is set using the API methods setDate and setLocalDate.