Basic subunit output

Add a skeleton to test the output.
Make the test do something useful
Refactor setup and teardown for test methods
Add copyright header to subunit_output
Include stderr and stdout in the stream
Add traceback to result
Support tags
Don't need to set inprogress so often
Add another test
Test capturing stdout/stderr
Confirm that email addresses do not appear in tag names

I've noticed them leaked onto the console though. Wonder what that's
about.
Handle undefined steps
Add a file which includes the steps

This lets us quickly see which step failed, and any steps which are
undefined.
Add subunit to the required dependencies
Add subunit to requirements.txt
Remove debugging comment that is no longer needed
It's python-subunit, not subunit
Python 2.6 compatible format usage
Merge branch 'master' into subunit-output
Add missing --with-subunit command