Add radio-buttons script and tests
Make setting of selectedClass to Constructor
Add focus tracking
Rename files to allow inclusion of checkbox code
Add support for checkboxes & begin tests for it
Merge both constructors into SelectionButtons

Includes:

- adding tests for all
- changing how focus is controlled for radios
Add documentation to README

Includes correction for naming of
selectionButtons.
Change selectionButtons to shorter form

As suggested by @edds.
Replace saved id of selected radio for its element
Change setup to getSelection
Move bindEvents method to BaseButtons

Includes the addition of the getEventNames method.
Remove all the _this variables
Add better option binding
Merge pull request #121 from alphagov/radio-buttons

Selection buttons