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
Remove duplicate grid row placeholder

This exists below, but with a descriptive comment - so the duplicate
placeholder above can be removed.
Merge pull request #138 from gemmaleigh/remove-duplicate-grid-row-placeholder

Remove duplicate grid row placeholder
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
Bump toolkit version from 1.7.0 to 2.0.0
Merge pull request #140 from alphagov/bump-toolkit-to-1.8.0

Bump toolkit version from 1.7.0 to 2.0.0
Remove ignore for ie

IE stylesheets then render the following where %site-width-container is
used:

margin: 0 15px;
margin: 0 30px;
margin: 0 auto;

So the auto margin overrides the left and right margins which are
otherwise set.
Merge pull request #141 from gemmaleigh/remove-ignore-for-ie

Remove $ignore-for-ie: true to set auto margins