Enhancing Bower integration

Currently, the way Aura is packaged doesn't make an easy integration with Bower:

Once you've `bower install`'d everything, you still need to `cd components/aura`, then go through the sequence of:
* `npm install`
* `bower install`
* `grunt build`

...which does not make Aura a "ready to go" component.

The purpose of this pull-request is:
* Reduce the build cost, by having `npm install` automatically executing a (locally installed) `bower install`
* Provide built versions of Aura right from the repository, in order to avoid the hassle of the steps above.

The built versions are named as `aura-VERSION.js` for non-minified content, and aura-`VERSION.min.js` for minified content.

The `build` task is still available to generate dev builds.