Use package.json's files field to cut down on what we publish

[jeffposnick]

We're currently distributing the following to npm:

$ ls
LICENSE         bower.json      docs            package.json        sw-toolbox.map.json
README.md       companion.js        lib         sw-toolbox.js

We should use the files field in package.json to limit what we publish.

Specifically, I don't think we need to publish the following:

• docs/
• lib/
• bower.json

[addyosmani]

I don't think we should distribute docs or lib as part of the package. Looking at the bower.json file, https://github.com/GoogleChrome/sw-toolbox/blob/master/bower.json#L4 seems like the only really useful hint and I don't think the lack of presence of this file is going to break anything. +1 for using files.

[jeffposnick]

package.json's main field lists lib/sw-toolbox.js, so I guess we should continue to shiplib/ as well. (It might be safe to just switch main to point to build/sw-toolbox.js, but we'd need to think more about how developers are consuming this library.)