DOC: existence docs and benchmarks.

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.