Refactor gruntfile and template compilation

It was impossible to test directives that use `templateUrl` with our former setup.

Templates are now loaded within their respective module and later compiled to a single file (`dist/templates.min.js` - the same as before)

Also led to a refactor of the Gruntfile in general for my DRY code regarding Arethusa modules.

`arethusa-cli` has been updated accordingly.