Fix issue where using the skip* family of decorators still ran the setUp and tearDown test methods.

This closes #86.

I make the testtools skip decorator set the __unittest_skip__ attribute on test methods, which makes them compatible with the unittest functions of the same name. 

I then change RunTest._run_core to look for that attribute and not run setUp if it's present. 
You might like to factor this out into a constant

require_27 = skipIf(...)

...
@require_27