Build the app in dev mode - unminified

So far we had no real chance to look at unminified code. Source maps don't work really well when debugging inside a browser.

Through the environmental variable `DEV` we can now build the application without minification. `DEV=1 grunt reload-server` would for exampe start the live-reloading server in this development environment.

This was solved slightly hacky - no changes outside of the `Gruntfile` had to be mode, that's why we opted for this reason.

Arethusa is still looking for files with a `min.js` extension - when the dev environment is active, we just serve a copy of the `concat.js` files with the `min.js` file ending.

Closes #390