Editable checkbox

Changelog:
* added rendering of checkboxes to dh-field directive
* added checkbox template to dh-field-editor directive
* editable checkboxes changes are now saved immediately
* added button like styling for checkboxes

Question: the checkbox template needs to be explicitly unhidden.
@leoasis do you happen to know the reason?

Note: styling inspired by http://plnkr.co/edit/BhTe4z?p=preview, but made a bit simpler (although the solution can be copied entirely). I also left the plain checkbox solution (commented) in case this doesn't feel right.