Github login

Support github login, close #521, related to #99.

- [x] Handle entirely new users signing up with GH
- [x] Handle existing, signed in, users linking their GH account
- [x] Handle cases where the username already exists.
- [x] Don't show 'link github' button if account is already linked.
- [x] Import/export gists using Github API
- [x] Revised tip/notification module for notifying user of important stuff (`tips.js`)
- [x] Better gist import url (`gist.jsbin.com/:user/:id => jsbin.com/gist/:id`)
- [ ] Test gist import with SSL support

Points for discussion:

- is every use case covered here? For example, people with different usernames on Github and JS Bin.