Redoes settings page

This PR does a few things:
This branch is currently UNSTABLE. The configuration iframe is non-functional and it has be tested not that much. Dabble at your own risk and definitely don't deploy to production.

* switches from Grunt to Gulp for the build process (why not?)
* restyles the settings page
    - breaks it out into it's own template rather than doing everything in .php files (hooray!)
    - adds a nice visualization of what your login form will look like
    - only shows relevant configuration options
    - adds a nice visualization of what your support badge will look like
* makes all of the settings persist over ajax through @jessepollak's new [Ajax Settings library](https://github.com/jessepollak/wordpress-ajax-settings/commits?author=jessepollak) (would love feedback on this)
* switches all of our JS to coffeescript
* includes [Bourbon](bourbon.io) and [Neat](neat.bourbon.io) for easier styling
* does a better job of asset management
    - minifies scripts in production, unminifies them in development
    - adds nice helper functions for adding assets to the page
* removes legacy use of $_SESSION for messaging (hooray!) and switches to using WP_Error and add_settings_error when necessary
* refactors code for determining whether passwords are disabled for a single user
* bumps version number to 2.0

TODO:

* [ ] better style the settings inputs (do we want to rely on the janky little checkboxes?)
* [ ] reintegrate the tutorial