Mouse click

I am going to need mouse click functionality for CRM corpus responses. I have not yet done all the event handling gobbledigook, but this at least provides the ability to poll the mouse button status. @Eric89GXL and @drammock, please take a look and comment when it's convenient.
I was a bit divided on this. Another option would be 1, 2, and 4, which are how pyglet defines ```mouse.LEFT```, ```mouse.MIDDLE```, and ```mouse.RIGHT```. But, having it be strings would be more like the keyboard presses, so I am fine with that.