Introducing Grunt

Closes #1897 

> Is there anyone around who could set up Grunt tasks to do the following?

> run jsHint on JS files
> run Autoprefixer on CSS files
> minify/uglify the sources
> concat all files

This takes care of what @nilshoerrmann asked for, except the use of jsHint.

I think jsHint should be used in your editor of choice while coding (or saving your code), not during the build process. I also prefer jsLint over jsHint. It's a lot stricter and forces a certain coding style which makes JavaScript easier to work on with other people.

If you still want it as part of the build process, it can easily be added, though.

I only added the assets Symphony includes by default. Core extensions like the devkits still have to be updated as well to use the new `../src/..` path.