Add polyfill for Function.prototype.bind

So that we can easily bind a `this` to function calls in event handlers.
Merge pull request #120 from alphagov/prototype.bind

Add polyfill for Function.prototype.bind
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.