Fix compatibility with browserify (Fixes #41)

This fix try hard to change almost nothing in 
the original implementation. 

This fix add two entry in package.json:
  - "browser" to override per-module some 
    dependency with an empty source file
  - "browserify" to add a transformation that 
    append a `module.exports = jsonldjs;` at
    the bottom of `js/jsonld.js` source.

And one dependency:
  - through

And one retro-compatible change on `js/jsonld.js` 
to allow the use of the bundled `js/Promise.js`.