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.
`1, 2, 4` might be useful because you could "or" them together. However, thinking of how that would be used (e.g., checking to see if the or'ed result is 3 to determine if left and middle are both pressed) is less explicit than saying `if 'left' in buttons and 'middle' in buttons`. It's more verbose, but also more human readable about what's being done. Might as well make use of Python's readability.