Proposed changes to JavaScript style guide

Per our discussion in the core team meeting today.

I've added a few new things here, including a proposed max line length and a rule that a literal object should either go on one line or be broken out per-property. 

I also tried to make some of the rules more explicit rather than depending on examples, but there is more work to do there and I'm not sure of the best way to write that up. I did that for the case of spaces in `prop:` vs `?`...`:` already. For example I think this is true, but a bit awkward to describe:

`{` when used as an object definition _can_ be on one line, `{` starting a block should always be followed by a newline.

Or maybe the example is all we need there.

@scottgonzalez @timmywil @gibson042 @gnarf37 @mzgol @orkel @jaubourg @mikesherov @rwldrn