JS/Python docs for `group`

I translated the docs for JavaScript and Python.

The code for the examples were in a "command-line" like style. 
We can't have the same thing in JavaScript and because it's easier to compare docs if they have the same format, I changed the ruby docs to not use a "command line" style for code.


I have a few more things I think we should change but I tried to keep the changes to a minimum here.
I would like to merge this thing first so:
- I can take care of https://github.com/rethinkdb/rethinkdb/issues/2076
- Iterate from there -- I'll open an issue for the things I think we should improve before releasing if possible

Ping @coffeemug -- could you take a quick look at this?