add compare methods to Results

[joep]

see thread starting Oct17 2010, Comparing models

proposal by Thomas Wiecki

http://bazaar.launchpad.net/~josef-pktd/statsmodels/statsmodels-josef-experimental-gsoc/revision/2186

For now I made two methods out of it compare_f_test and
compare_lr_test both attached to regression results
I think, compare_lr_test will go higher up in the class hierarchy so
that it is also available for maximum likelihood models.

I like the more explicit names, but shorter names might be possible

In the simple example, compare_f_test produces exactly the same result
as f_test with the corresponding restriction matrix, as it should for
linear models. Likelihood Ratio test results are close to f_test but I
don't have an exact test for it yet.