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.
A bit late of a reply (as you already adapted it), but I don't think it is really a problem. It is also the way we do it at the moment in the other section of enhancingperf.rst (eg with the cython http://pandas.pydata.org/pandas-docs/stable/enhancingperf.html#adding-type). 
Because now you have also the problem of how to keep this up to date? (as there will be changes to performance in the future). Test it before each release and update the numbers?