Add Gengo API tests and skip_if infrastructure

This adds some skip_if infrastructure for skipping tests if an
expression evaluates to True.

It also adds a couple of tests that actually execute the Gengo API calls
rather than executing against a mock Gengo API.

I should have done this when I did the machine translation changes.

r?
I didn't spend a lot of time on looking into whether there are better ways to do this. I mostly just wrote it. It's entirely possible there are better ways to do this out there.

Incidentally, the unittest module has similar things:

https://docs.python.org/2/library/unittest.html#skipping-tests-and-expected-failures

The difference is theirs evaluates the condition at the time of definition--not at the time it's executed. Mine runs it at the time of execution. I'm not sure I need that, though.

Regardless, that skipIf stuff is not in Python 2.6. :(