Check for the Resource dir before building

This is the first commit of a series in this pull request. Please don't merge anything until the checklist is complete AND you're happy with the commits. :wink: 

What I plan to do, please comment and improve this list as you want :
- [x] Check all the fs calls
  - [X] The Resource directory could be omitted
  - [x] node-webkit could be in ~/Applications (see #2)
  - [x] separate even more the constant path to the model
  - [x] use platform-independant paths manipulation
  - [x] validate that nothing else can be absent