Switch from PyGame to PySDL2

[evoggy]

To minimize the dependencies and increase the functionality for the input devices we should switch to PySDL2.

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• Bitbucket: https://bitbucket.org/bitcraze/crazyflie-pc-client/issue/99
• Originally Reported By: Marcus Eliasson
• Originally Created At: 2014-01-20T08:54:55.066