Threaded Execution

WIP (dont merge)

Done so far:
- by default serial execution w/o threads
- threaded execution of tests if requested
- uses ``argparse`` (supports ``--help`` and ``--version``)
- ``group`` argument accept multiple groups
- new ``threads`` argument
- no support for python 2.6 any longer
- increased version to 2.0
- use future print as guido likes it
- other minor code modernizations

Problems to solve
[ ] output of threads is mixed
[ ] robot tests are not running on different zserver ports 
[ ] check what happens with xmltestreports?