Modified the datepicker button a bit

- Uses the calendar icon instead of "C"
- In math4 the button is attached to the input.
Merge branch 'release/2.8' of http://github.com/openwebwork/webwork2 into newbutton2

Conflicts:
	htdocs/themes/math4/math4.css
Merge pull request #216 from goehle/newbutton2

DateTimePicker Button