compare the number of disabled tests to the total number of tests

Previously, ```GHTestGroup``` would compare the number of ```children_``` to the number of disabled test methods to see if there were any to run.

But in the top level ```GHTestGroup``` object, ```children_``` is a collection of test classes, not test methods, so the comparison didn't seem right. I'm now comparing the number of disabled test methods to the total number of test methods - and my freezing problem has gone away.

(See issue 46 for more details. In my particular case, I had several test classes - some of which were just base classes for common functionality, so didn't contain any test methods themselves - and the application appeared to freeze if I chose to run just one test method. May be because the number of test classes was greater than the total number of test methods in those classes.)