add browser implementations

closes #4

Adds the browser implementations for buffer and reader.
[browserify](https://github.com/substack/node-browserify) is used to bundle the required files into a single osm-read-pbf.js.
Install with 

    npm install -g browserify

Run with (npm script):

    npm run browserify

Adds browser example ``example/pbf.html`` and extends the browser test (needs the bundle). Because both are using Ajax to load the .pbf file, there are restrictions from file:// URLs - Firefox works, Chrome needs the ``--allow-file-access-from-files`` parameter or HTTP-serving the directory.

Note: I force rebased this branch, should be fine, but in case of any issues when merging, let me know.