Refactor to allow selector strings to be passed in

Includes the loss of GOVUK.RadioButtons and GOVUK.CheckboxButtons.
rewrite tests for GOVUK.selectionButtons & add new

The old ones were too coupled to the implementation. These are written
to better reflect the behaviours expected.
Refactor selection-buttons for readability
Add an explanation of the new interface to README.md
Tidy up markSelected method

Includes fix for misplaced comma.
Change 'bind' prefix to 'add' for all methods
Merge `setEventTypes` into `addEvents`
Refactor to allow use of GOVUK.SelectionButtons

Allows storage of returned instance & for the destroy method (previously
`GOVUK.selectionButtons.removeEventsFor`) to work on that instance.
Add documentation for new interface

Includes a note on dealing with deprecated functionality.
Merge pull request #139 from alphagov/add-document-level-events-to-selection-buttons

Add document level events to selection buttons