Initial work of Grunt.js-ifiying the repo

Hey, first of all: very nice stuff! I like the idea of rendering the sequence diagram entirely in the browser :+1: 

I've seen in the README that you plan to use Grunt.js instead of the Makefile, so I gave it a try. I'm not sure if I moved the functionality correctly and I couldn't find out how to test my changes, also I plan to update the README and remove the obsolete Makefile.

I just wanted to open this PR to get some early feedback.

This will replace the Makefile like this:

``` shell
# install grunt
npm install -g grunt-cli

# install dependencies used in the Grunfile.js
npm install

# execute default task, which creates the sequence-diagram-min.js
grunt
```