Integrate NodeJS+browser builds & use grunt through NPM

This PR is related to #66, #72 and #53.

This is a minimalistic work, made in order to merge nodejs & devel branches, by integrating both of them in the grunt build and package.json.

* ```npm install``` now installs grunt and karma as well (not phantom yet, but it's easy to add as well), and builds both nodeJS and browser versions of the lib in ```build/``` directory
* ```npm test``` now runs both nodejs (using mocha) & browser (using karma/phantom) unit tests. Both are working (yet we have a small warning from eventemitter2 in the console for the corresponding test)
* ```npm publish``` should now run (untested) a build prior to publishing to the repository, so it's not error prone.

I did not work on Travis-CI integration, and I've seen there is a little more work in order to revert the difference between nodejs branch and devel one for merging everything. Still, it should be pretty easy to do !

I've tried to modify as few files as I could in order to facilitate merge.