Fixed the `compass:production` and `styles` tasks

Now, there will be only one SASS stylesheet non-partial, `app.scss`,
since there is no point for a Single Page Application have more than
one stylesheet if it is not loaded asynchronously. Another non-partial
will be compiled to `dist/` when performing production build, which is
not the ideal, so it must be avoided. However, when needed, the
Gruntfile should be modified to better fit this case, as put all
stylesheet under `dist/stylesheet` directory.