Fix for issue 18: Allow setup.py to work without required twisted to be preinstalled.

Move version to its own module so that version can be defined in one place but setup.py does not need to import crochet to get it. This removes twisted as a requirement for setup.py to work. Instead setup.py just execs the version module to get the version.

Also includes a tox.ini which can be used to test for install problems like issue 18.