Rethink project structure

The way this repository is currently structured is kinda restrictive (e.g.: we can't [change](https://github.com/h5bp/html5-boilerplate/pull/1550) existing files or [add new ones](http://editorconfig.org/) that can be useful in development but are not be useful to our end users). Also, manually updating the external components like jQuery, is kinda time consuming.

So, to address the above I made a [simple proof of concept](https://github.com/h5bp/html5-boilerplate/tree/rethink-project-structure) (the code is WIP).

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Note(s): 

* This change will also addresses the problem discussed in https://github.com/h5bp/html5-boilerplate/pull/1550 (the build script can just generate an archive with only the needed files (the content from the `dist` directory), [archive that can then be attached to the release](https://github.com/blog/1547-release-your-software)).

* If we decide to go on this path, the [server update process](http://demo.html5boilerplate.com/) also needs to be updated. 

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Thoughts? (Cc: @h5bp/owners)