Copying login modal from shippable.com.

This copies the login modal and icons from shippable.com to the docs.shippable.com login button.

![screenshot from 2014-05-29 15 56 42](https://cloud.githubusercontent.com/assets/5492015/3125087/e19eafce-e784-11e3-9d77-bf2124e4dfee.png)

Shippable/pm#700