fix some tests/examples for python 3 or if missing statsmodels

One of the example tests currently fails on python 3 because it uses print statements instead of functions. If we support 2.6+ we can use `__future__` imports to make them pass on both 2.6+ and 3.x. I've added a print statement to the glm_robust.py to explain why it exits if statsmodels is missing.

The glm examples were missing imports for sys in the case that statsmodels is not installed.

There was an unused import of SkipTest in test_glm.py. We should call it instead of exiting if statsmodels is not available.