Grunt build process

Replaces Makefile with Grunt that fits better into web developers workflow. This PR contains the following changes:

* Adds `Gruntfile.js` and `package.json`
* Removes `Makefile` and `version.txt`. From now on, version number must be managed in `package.json`. Grunt takes care itself in order to write correct version numbers where needed.
* Delete versioned .js files from repository, new ones built with Grunt are still under `dist/` directory but i think they should be removed from git respository as well and kept under releases in GitHub

We also chould add `bower.json` file generator based on information stored in `package.json`