Add support for watchify

[jesstelford]

By not passing back any fileData, we trigger module-dep's readFile function, which in turn emits the 'file' event that is required by watchify to correctly trigger an 'update' event, allowing us to rebuild.

(This was a doozy of digging through events to figure out!)

[jesstelford]

Actually, on further thought - does this actually achieve anything? Using watchify means that rebuilds are very fast due to a different caching mechanism used internally. And given this change forces rebuilding on every run, it wouldn't be useful as a cache across runs (or across modules that share the same cache key object).

Thoughts?

[martinheidegger]

Oh my...

the first thought that comes to mind is that watchify should use the new cache API ;). This PR though destroys all the value we get from the fs cache and I think we have to take a different approach to fix that.

[martinheidegger]

This is actually an issue in module-deps. I am preparing a PR to fix it.

[martinheidegger]

In case you didn't see it: I opened browserify/module-deps#127 that should fix this issue. It would be lovely if you could review/test it.