Introducing Grunt #1897

This takes care of what @nilshoerrmann suggested in #1897 except the use of jsHint.

I think jsHint should be better 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 multiple 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.