Add contextEvent mapping for all wirings (except values)

As discussed in #50 I'd like to propose to add `contextEvent` mapping for all wirings.

Two caveats:

1. I didn't add automatic context event mapping for values. It would be possible, but there's a problem with value wiring I'll be describing in #58 and thus left it out. Depending on how 58 goes I can still add it.
1. The prototypes subject to contextEvent mapping need to extend Backbone.Events (or at least fulfil `validateListen`)

And some food for thought: an instance is responsible for removing these context event listeners itself. This is something we definitely need to point out. Or find a workaround for. ATM, i.e. in the current code base, this _is_ a problem for all views bound with `bindContext` as well.