New Option Type: Date

Output Screenshot: http://grab.by/Bq9k
Input Screenshot: http://grab.by/Bq9m

Notes:
- I didn't have time to make it work within theme customizer, but it
shouldn't be difficult to add.
- Tried to keep it as simple as possible
- JS Libraries used: jQuery UI Datepicker (included in WP) and a
Timepicker add-on for it (
http://trentrichardson.com/examples/timepicker )
merged
Merge branch 'ardalann-master'