Added support for custom TextTestResult class.

Hello,

This small patch adds support for custom result classes, similarly to how pyut TextTestResult does since 2.7.2.

(Disclaimer: this work was done while at Cisco Systems LTD and all the required authorizations for pushing upstream were obtained through the internal open source patch channel)
I don't believe it will.  Looking at the source for Python's unittest library, it's only used in _makeSuite().  One could argue that we don't even need to pass it down, since we're overriding `_makeSuite()`.  The way I look at this is that you're basically backporting the feature, since you've really got everything already in place to do so.

Also, I think it would be more disturbing to set the resultclass and then have something different happen on Python < 2.7.2.