Experiment data etl + api endpoint

[wlach]

Example query:

http://localhost:8000/api/experiment/?measure=main_crashes&interval=200000&experiment=pref-flip-activity-stream-58-beta-pocket-personalization-bug-1425490

Example response:

https://gist.github.com/wlach/751f81dacf278e1b8ac4f8eee6e4b49c

[wlach]

@robhudson ok, I think this is ready for actual review now. hopefully the above is helpful for understanding what this does

[robhudson]

Is it possible to document the API, similar to what we do on EV? https://github.com/mozilla/experiments-viewer/blob/master/docs/api.md

[wlach]

I will add some docs in a subsequent PR. I like the approach of a simple markdown document, at least for the initial stages.