Further refactoring for licence model.

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
Validator updates, close #39. Last bit of licence refactoring.

Validator updates:
- Licence used/defined corrected
- ExtractedLicence pointers are validated only once

Refactoring of licences:
document.ExtractedLicenceInfo is now document.ExtractedLicences

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>