adding sourcemap support to browserify

This still depends on my fork of browser-pack.
So we first would need to merge [this](https://github.com/substack/browser-pack/pull/4), publish as new version and the change the dependency.

At this point only a `--debug` flag is supported which inlines sources by default and puts sourceRootURL at the root of the file system.
In the future we should support more flags, to opt out of inlining sources (keep bundle smaller) and to point sourceRootURL wherever we like.

I'll add these in a later PR, at this point I wanted to get the minimum functionality in.