ENH: Add vbench

Closes #936 and supersedes #1070. I just rebased and updated @TomAugspurger's code to run on our docs build box.

You can see the results here. No link from main docs yet AFAICT. 

http://statsmodels.sourceforge.net/vbench/

Comments? It might be nice to use the same CSS as the main docs. I have no idea why the performance looks random. Likely some background process on the box. Don't know how useful this will be unless we really identify long(er)-running code with hotspots that is gives fairly reproducible numbers.