Contributing/style guide

[euank]

Pretty much irresponsible not to have one these days.

Note, this requires us settling on a JS style which we've avoided.. or documenting that each module gets to pick whatever it wants to (please no)

[brhoades]

Let's do eslint with airbnb.
https://github.com/airbnb/javascript

[euank]

Thanks for the recommendation, LGTM @brhoades.

I made an initial shot at the eslint part over here: #171

I'm leaving this open since contributing.md still needs to exist regardless.