ENH: Fixes and tests sign_test. Closes #642.

Moved sign_test out of the sandbox and test it. Clean up the Describe class a little bit. This needs some attention. I think it's an obvious candidate for just using a DataFrame internally, but I'm not going to look at this now. It's low-hanging fruit though and deserves its own ticket probably.

Of general interest, I added `statsmodels.tools.decorators.nottest`. We can use this to mark things that are named `xxx_test` as not a test for nose. It's safe, so that if users don't have nose installed it's just a dummy decorator. Nose is still a soft-dependency.