Issue #24: fixes for running under cricket under Python < 2.7

_ttk_ was added to standard Python distribution in 2.7, so it wil fail under earlier versions:
http://bugs.python.org/issue2983

To mitigate this, we check for installed Python's version and add _pyttk_ if necessary.

We use _unittest2_ in place of unittest because it contains backported changes from _unittest_ in Python 2.7 and cricket relies on them.