making ZeroClipboard.currentElement private
Making sure that we only register an event name once

In response to concerns #67 I am removing the functionality to register
multiple functions for the same event name.

This will not register a new eventname if that handler already exists.

@JamesMGreene
Merge branch 'master' into client-contstructor

Conflicts:
	ZeroClipboard.js
	ZeroClipboard.min.js