Make the SwatDetailsViewField's header title come from a method and update the vertical field to use it.

This fixes two issues in the SwatDetailsViewVerticalField header:
 * It now respects the show_colon property.
 * It now shows a &nbsp; when no title is set, just like SwatDetailsViewField
Add docs.
Merge pull request #23 from isaacgrant/show-colon

Make the SwatDetailsViewField's header title come from a method