Fix --diff option

This is a fix for ticket #66

I have also updated the version so that a new release can take place.

I have refactored the way diff file is read to help with testing.

It looks like the current code is already using something similar to buildbot configuariton.

I wrote tests for the diff command.

Please let me know if you thing that this looks good.

This is of great help when I work on Twisted and want to use twistedcheker on my local computer.

Thanks!