Bug with listeners in Selection.teardown method

[vmstarchenko]

Bug report

I am not sure but visually there is a bug with Selection.teardown method. Everywhere in this file variable _listeners is used but in teardown method it doesn't starts with underscore:
this._listeners = Object.create(null)
vs
this.listeners = Object.create(null)

(https://github.com/jquense/react-big-calendar/blob/master/src/Selection.js#L47 vs https://github.com/jquense/react-big-calendar/blob/master/src/Selection.js#L102)

It might cause a problem with event listeners unbinding.

[cutterbl]

@vmstarchenko Thanks again for pointing this out. It has now been resolved.