Project reorganization

Hello,

I spent some moments on refactoring the project in order to improve structure and dependencies management.

- External libraries now imported with CocoaPods (OpenSSL, QR Encoder, ZBar)
- Xcode groups mapped to physical directories (Models, Views, Controllers, Entities)
- Libraries renamed to "External" (only Reachability.*)

To be honest, I haven't had the time to test the change thoroughly but no file was actually modified, so I don't really think anything broke.

Cheers