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.~~ but since dependencies aren't resolved for values it seems to me that contextEvents shouldn't be mapped either.
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.