Interchange folders dev/ and /app

The project uses *dev* as a term for 'in development mode' through out the project, e.g. 

```javascript
// Views
grunt.registerTask('buildViews', ['jade:dev']);
```

However the project has different use of the term *dev* in the folder structure. I simply changed the  folder `app/` → `dev/` and all related references. It just makes it much more consistent with the rest of the project.

Cheers!