Add skipTest method to nose plugin to mask method in TeamcityTestResult.

The method in TeamcityTestResult takes a test instance and a reason, but
when nose calls skipTest on a plugin, it only passes the test instance.
So, a limitation of this solution is that the text for the reason won't
show up in the teamcity report.
Merge pull request #1 from Points/log_skips_in_nose

Add skipTest method to nose plugin to mask method in TeamcityTestResult.