Let's use Grunt instead of shell scripts

Available grunt tasks
* `grunt css:debug` Build CSS files from LESS files, without compression
* `grunt css:release` Build CSS files from LESS files, with compression
* `grunt css` Same as `grunt css:release`
* `grunt js:debug` Build JavaScript files, SourceMap files and link them together
* `grunt js:release` Build only JavaScript files, without SourceMap files
* `grunt js` Same as `grunt js:release`
* `grunt debug` Same as `grunt js:debug css:debug`
* `grunt release` Same as `grunt js:release css:release`
* `grunt` Same as `grunt release`

See the discussion at http://www.concrete5.org/developers/beta/beta_discussion/what-about-switching-to-grunt/