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.
This is a little bit baffling to me ... seems like `pyglet` must expose a nicer mouse API than this. I assume you've looked though, so if this is the best we can do, so be it. You could also do `np.where(self.win._mouse_buttons[1:])[0]` to get an index array of all the `True` buttons (and why does it need to start from one??? Silly Pyglet).