Merge the 200Creative/spree_bootstrap_frontend gem into Spree.

Hi everyone,

This PR attempts to integrate 200Creative/spree_bootstrap_frontend into spree_frontend. If this PR is accepted, spree_bootstrap_frontend will become a part of Spree, and the extension will be deprecated.

This PR isn't complete and isn't ready for merging just yet. We've decided to open a PR now in the interests of visibility and to get feedback on the PR as soon as possible.

__What's left to do?__

* [ ] Integrate spree_bootstrap_frontend with spree_auth_devise
  * [x] Pull out spree_auth_devise specific code into an internal gem
  * [x] Merge latest changes from spree_auth_devise (up to 761a9d5)
  * [x] Make specs pass (up to 761a9d5)
  * [ ] Merge spree_auth_devise specific code into a fork of spree_auth_devise
    * [ ] Manual regression testing
    * [ ] Create PR with spree_auth_devise
      
* [ ] Integrate spree_bootstrap_frontend into spree_frontend
  * [x] Pull out spree_frontend specific code into an internal gem
  * [ ] Copy bootstrap-kaminari-views into spree_bootstrap_frontend
  * [x] Merge latest changes from spree_frontend (up to b07b838)
  * [x] Make specs pass (up to b07b838)
  * [ ] Merge changes into spree_master_with_bootstrap_fork
    * [ ] Merge gemspecs
    * [ ] Merge Gemfiles
    * [ ] Merge CSS & JS
    * [ ] Remove installer generator
    * [ ] Update frontend/README.md to better reflect the merging of both repos
    * [ ] Move decorator code into real classes
    * [ ] Manual regression testing

* [ ] Integrate spree_bootstrap_frontend into spree_core
  * [x] Pull out spree_core specific code into an internal gem
  * [x] Merge latest changes from spree_core (up to b07b838)
  * [x] Make specs pass (up to b07b838)
  * [ ] Merge changes into spree_master_with_bootstrap_fork

---

Switches out Spree’s entire frontend for a bootstrap 3 powered frontend.

This has several large advantages:

- Fully responsive - Mobile, tablet and desktop. With custom grids for each, collapsing elements, and full HDPI support. Current spree only goes half way.
- Just 44 lines of custom SCSS, replacing 1328 lines of undocumented spree CSS. Plus most of these lines only add some visual style to the header and footer and can be removed.
- The entire frontend can be easily customized: colours, grid, spacing, etc, by just overriding [variables from bootstrap](https://github.com/twbs/bootstrap-sass/blob/master/vendor/assets/stylesheets/bootstrap/_variables.scss) - giving a custom store design in minutes.
- Bootstrap has some of the most [robust documentation](http://getbootstrap.com/css) of any framework, and a hugely active community. As this port uses only default bootstrap it means that entire spree frontend layout is documented by default.
- Sites like [bootswatch](http://bootswatch.com) allow for one-file bootstrap drop-in spree themes.
- Lots of [spree community will for bootstrap](https://groups.google.com/forum/#!searchin/spree-user/bootstrap/spree-user/B17492QdnGA/AF9vEzRzf4cJ).
- Though this uses ‘full bootstrap’ for simplicity, you can remove the unused Bootstrap components you don’t require for minimal file sizes / weight.
- Bootstrap is one of the largest most active open source projects out there - maintaining an entire framework just for spree makes little sense. Forget about cross browser bugs. Woo!

Due to the markup changes this is a breaking change. Any extensions that use selectors against the original markup will break.