extract zlib dependency

part of #4

Extracts the ``zlib.inflate`` call into separate implementations, mainly to use the ``blob.raw_size`` in the browser implementation. Which implementation to use is determined by testing for the existence of ``self`` (Browser+Web Workers) and requiring the appropriate file - haven't found a better way.

Adds the [imaya/zlib.js](https://github.com/imaya/zlib.js) library for the browser as an optional dependency, install with: 

    npm install

Adds an ``test/index.html`` for testing in the browser, requires latest should: 

    npm update