Based on https://github.com/ccampbell/mousetrap.
Cleaned up to fit our coding standards, pass JSHint, and assume
jQuery's fixes where possible (e.g. no need for an addEvent
utility, no need for filling e.target, e.which, etc. cross-browser
which jQuery.Event already does).
Initially all were local functions in the constructor, but to
allow some customisations in subclasses moved various methods
to the prototype instead and marked with @private.
Really, only .register() must be called from the outside. The rest
assumes normalisation etc. or might break things if called
incorrectly.
Change-Id: Ic69a3c70759052094aefbeab623d885f8b118e14
