Added js style guide

This is a first draft of a JS style guide that @robertkowalski and I put together. Feedback very welcome!
2 space tabs is almost ubiqutious in javascript world, in PouchDB we also use an 80 char limit, we actually just link to http://nodeguide.com/style.html for our style guide, (https://github.com/pouchdb/pouchdb/blob/master/CONTRIBUTING.md#guide-to-contributions) most of it we enforce with jshint