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.
make this initialization section simpler, e.g.

```
l = range(1000000)
l_series = Series(l,name='data')
df = DataFrame(l, columns=['ID'])
```

e.g. dont' need the ``pd.``; also I don't think its necessary to show df