Ditches gulp, submodules, moves assets into its own dir

This PR:

* Unminimized static assets and removes `gulp`. This also removes `node_modules` and `package.json`.
* Freezes `jquery.dotdotdot` into the repo, rather than use a git submodule. This removes `.gitmodules`.
* Moves all static assets -- CSS, JS, images, and fonts -- into an `/assets/` dir, to clean up the project root.
* Fixes `/developer/`, which didn't get migrated properly in #135.
* Renames `/styles` to `/css`, for terseness and to match the other directories in naming them after their format.
* Moves the IE8-specific assets into `/ie8` subdirectories in `/js` and `/css`, so their purpose is clear and it's easier to concentrate on the feature-specific files.
* Updates the README to reflect all that, and to note that Jekyll 2.X now [requires Node](https://github.com/jekyll/jekyll/issues/2327#issuecomment-55337023) (!) as a dependency. This is being removed in Jekyll 3, and I'm going to see if I can remove this dependency by using a stable enough pre-release version of Jekyll.

Things to do next (see #135 for a more comprehensive list):

* Remove jquery.dotdotdot altogether, even if it means just doing a hacky JS line ourselves.
* Remove jQuery altogether.
* Find another way to validate Google Webmaster Tools, and besides DNS (since we don't have easy control over that).

We're still waiting on a certificate before moving our actual production domain over to the new setup.