When django-evolution evolve hits a problem, it currently dumps out the error string, and then uses sys.exit to escape.  This is problematic when attempting to leverage django-evolution programmatically, since we really don't want to have to catch the SystemExit exception.

The preferred way of signaling when something has gone wrong with a Django command is with the CommandError exception.  I've replaced all instances of sys.exit with this exception.

I've also taken the code out of handler, and put it in a new method called evolve.  This will make it much easier to call evolve programmatically, and to catch exceptions.

All tests pass.