Refactor Pure metadata gathering/processing and Add visual list of Grid Units

This makes Pure's site depend on Pure via Bower, always. This allows the website to gather and process metadata about Pure on startup, like the version, filesizes, and current set of grid units.

Since gathering this metadata is done async, a new middleware function has been added which will do all the async metadata gathering and processing on the first request, _only_, after which the data is cached.

There is now a visual list of Pure's built-in Grids units.

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### Improvements to Local Pure Testing

The website is now really smart in how it can integrate with a local working copy of Pure. Simply use Bower to link your local copy of `pure` to `pure-site`:

```shell
$ cd pure/
$ bower link
$ cd ../pure-site/
$ bower link pure
$ npm start
```