Add convenience method to bind Emittery methods to own instance #30

EventEmitter in Node.js is usually subclassed, but I don't like this pattern, and I don't think we should recommend it. It's better to have an internal Emittery instance and just explicitly expose some of the methods.

We could make this even easier, by providing a convenience method to bind the Emittery method to the instance:

class Foo {
    constructor() [
        this.emitter = new Emittery();
        this.emitter.bindMethods(this, ['on', 'off']);
    ]
}

The above would quickly expose the on and off methods, but nothing else.