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 an SSL certificate before moving our actual production domain over to the new setup.