Feature/python 3

Introducing: The Python 3 version of Pelita.

Most went well with 2to3 but a few additional fixes were needed with string conversions (mostly for the socket I/O) and newly introduced name conflicts.

Thanks to our better-than-average coverage, I’m also rather confident that there are not too many bugs left.

Notes:
— The compat.argparse has been retired
— I’ve added a python3 shebang but I’m not sure this is correct and future-proof (e.g. travis complains about it)

Are there any great Python 3 features we could immediately take advantage of?