DOC: existence docs and benchmarks.

![enhancing_performance](https://cloud.githubusercontent.com/assets/1731217/3224419/636f5dfc-f034-11e3-90e8-1469a162b674.png)

I've included some documentation on existence-type associations as requested by Jeff in [this question](http://stackoverflow.com/questions/23945493/a-faster-alternative-to-pandas-isin-function/) over at Stack Overflow.

The content includes a couple of plots generated by the script bench/bench_existence.py but I was not able to find any automation that executed those bench/bench_*.py scripts, so I may be missing a hook somewhere. That script also takes a couple of minutes to run.
I think you can use `join` to achieve this, but then you should do something like `df.set_index('ID', drop=False).join(..)` in a more general case