Re-use #off in #connect and add context argument to #off

Resolves issue #10

Objectives:

* Make it easy to extend oo.EventEmitter by only overriding #on and #off
* Add context functionality to #off

Changes:

src/EventEmitter.js
* Add optional context argument to #off, which defaults to null, just as it
  does within #on
* Reuse off method for actual removal of bindings inside #connect