Minor refactor in Gruntfile
Add grunt-contrib-copy
Add grunt-contrib-clean
Use concat files in dev mode

We are still using *.min.js files in the application, so we don't have
to change anything there. But when we build the system in dev mode, we
only copy to *.concat.js files to *.min.js instead of really minifying
them.
Guarantee a clean build on server start

Clean the dist folder, so that grunt-newer has to rebuild all files in any
event.