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.
Can you state a bit more what you exactly understand by 'existence'? In general + in practice the exact operation you want to test in the examples. As it is not fully clear to me (thinking of myself as a not so experienced user) what you actually do in the different examples.