Refactor the selector column plugin into a mixin
Move documentation from the GH wiki into the repo.
Update Selector documentation
Further refactoring to Selector mixin

* Fix a bug related to multiple columns
* Move more functions into the prototype so they can be overridden
* Make Selection and Selector consistent WRT disabled row handling
Additional Selector tweaks and test cleanup

* Use late binding for better extensibility
* Tweak name and type of 2nd argument to renderSelectorInput
* Move selector check outside of _configureSelectorColumn
* Add flag to avoid needlessly calling _updateHeaderCheckboxes
* _changeInput -> _changeSelectorInput
* Minor reformatting
* Update remaining instances of selector: true to "checkbox" in tests
* Remove grids in test page showing spurious use cases with checkbox + radio
Tweaks to Selector docs
Update docs based on latest wiki content and 0.4 changes
CHANGES.md: Note Selector's conversion to mixin