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.
Instead setup.py just execs the version module to get the version.
Added simple test to catch install issues like #18 in future
Merge pull request #20 from cscutcher/issue-18

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