Renamed 'Assign' button to 'Assign / Unassign'

The assign button on the deployment page actually removes hosts from a
hostgroup as well as adding them.  This behaviour is not obvious since
the button is named 'Assign' which suggests add only.  Renaming to make
the behaviour more obvious.
Merge pull request #125 from mtaylor/rename_assign_button

Renamed 'Assign' button to 'Assign / Unassign'