Support browser-targeted build.

This adds two artifacts to the build: uncompressed (development) and minimized (production). Both can be loaded with Require.js or similar and used as a global var `Immutable` in browser.

Artifacts names:
* `immutable-x.y.z.js`
* `immutable-x.y.z.min.js`

Placed to `dist/browserify`.
This isn't necessary, right? package.json is always used by default