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 left it only in this path so that the default TextTestResult would be used with version < 2.7.2. Wouldn't having support for custom result class in the Nose TextTestRunner but not in the underlying unittest TextTestRunner potentially cause problems? If you think it is safe I'll happily change it.