Multi item licence

Changes:
- licences quantity assigned to (asset|user)
![image](https://cloud.githubusercontent.com/assets/351535/4457415/b844f976-4896-11e4-8fe3-f366598c4a3e.png)

@xliiv @quamilek please review.

Doesn't this restriction breaks the whole concept of 'adding only mixin *AssginLicenceMixin* and works'?