Issue #375: Minify CSS

I've implemented minification of the CSS using grunt-contrib-mincss.
I moved the css file to src/css and updated the Gruntfile tasks for build, prod, version and debug - taking the appropriate action for the CSS files.
I accidentally made this change from my master - so it includes a change to ng-grid to fix the row calculation error on window resize.
Files to take are:  
*  Gruntfile.js and 
*  package.json as well as 
*  the relocated ng-grid css (into src/css)

To see it in action: http://plnkr.co/edit/9GBpJB?p=preview

hth