add multifieldhistogram panel

This is a new implementation of https://github.com/elasticsearch/kibana/pull/374 but in a different panel and rebased on master.

It does more or less the same thing as https://github.com/elasticsearch/kibana/pull/374
- you can plot different values taken from different fields, and from different queries
- you can give a new alias and color to this plotted value(s)

![b003](https://cloud.githubusercontent.com/assets/5235401/3243726/d3fb976e-f15d-11e3-9f70-119007994c31.png)

My use case: I have documents which contain the percentage of cpu utilization of some machines {idle:30, user:10, iowait: 4, ...} and I want to plot this on the same histogram. I wanna have a different color/alias for each of these fields, cause I'm not interested about the query it comes from.

There is room for improvement in the UI as well as in the JS. Let me know what you think.

Cheers,
Benjamin