Use hash ids for pack output

This fixes #13.

Instead of ints, this uses the `browserify._hash` method on module ids in both entry and common bundles. Common dependencies are added to `browserify.exports` to ensure their hashes are exposed (simulates using `-x` and `-r` to generate factored output).

The cost of improved accuracy is a few extra bytes per dependency.

Note that this requires browserify ^3.30.5, due to the use of `._hash`. I took the liberty to update some dependencies in this PR, but I can revert that if you like.