Add ability to display custom date/time format from schema seperate from...

... cinnamon desktop wallclock, this matches the other clock shaped things in the cinnamon desktop

Depends on linuxmint/cinnamon-desktop#16 for schema and not dependant but useful https://github.com/linuxmint/Cinnamon/pull/3404 for settings

![screenshot from 2014-07-15 03 01 29](https://cloud.githubusercontent.com/assets/1816681/3582625/bcfe03f6-0bfe-11e4-8abb-6a08e08b0cad.png)