Modified DSL to support radio group attribute

Added tests and code to support :group for radio buttons at the DSL
(non-SWT) level.
Added basics of RadioGroup

Added starting structure of RadioGroup.
Refactored SelectionListener

Refactored SelectionListener to common base class.
Created separate Radio class

Made separate Radio class since it now has group functionality.
Added group attributes

Added group attributes and moved radio_groups to RadioGroup class.
Added testing and refactored location or radio groups

Added testing for RadioGroup and Radio and moved all_groups to
RadioGroup.
Added additional Radio tests

Added radio tests for group=.
Updated Radio and RadioGroup based on comments

Modified Radio, RadioGroup and associated tests based on Github
comments.
Modified RadioGroup and associated tests

Made RadioGroup enumerable and added tests for
select_only_one_radio_in_group
Merge pull request #444 from rjack1201/radio_grouping

Radio grouping