Better Grunt + Bower -> Removing Normalize.css from `src/`

This is a revamp of the Grunt + Bower setup to remove the need for checking Normalize.css into the `src/` tree. As a result, `forms-core.css` has also been removed, because Pure really does depend on Normalize.css being on the page.

The Gruntfile has also been simplified, running `grunt` will install any Bower dependencies, run tests, and build Pure. The `watch` task first runs `grunt` (default), then observes changes and runs the `test` and `build` task, no Bower installing (so it's faster).

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The also updates our npm dependencies to their latest versions.