Reduced #off polymorphism, added #unbindAll

It is no more possible to unbind all handlers from one or more events.
Also, it is no more possible to use Emitter#off to unbind every handlers from every events, you have to use #unbindAll instead.
Updated #off and #unbindAll related tests
Added gjslint in Gulp lint task

Also fixed some related broken rules.
WIP/BRK: Trying to replace Binder by a parent system
Fixed: Disabled emitter no more emits any event

The test on the _enabled property was missing in the #emit method.
Cleaned a bit unit tests
Added #child and #kill methods

The #kill method destoys an instance.
The #child method creates a new instance of Emitter thas is somehow bound to the parent:
 - When the parent emits an event, the children will emit the same later
 - When a child is killed, it is automatically unreferenced from the parent
 - When the parent is killed, all children will be killed as well

Also added unit tests and removed the #binder related ones.
More cleaning in unit tests
Added the #once method, and options for #on

It is now possible to give an object of options to the #on method. The only recognized parameter implemented yet is the "once", which makes the bound function being unbound after the first time it is executed.
Also added a shortcut, the #once method, that will call the #on method with an object of options with only the "once" flag set to true.
Finally, some very basic unit tests about all that have been added.
Merge branch 'v2-proposal'

Conflicts:
	emmett.js
Released v2.0.0

 - Removed #binder and added #child
 - Added #enable and #disable
 - Added #kill
 - Added #once and options for #on