Contracts implementaion

[AnatolyRugalev]

Hi, @crisu83

Just wanna let you know about my fork. It was hard for me to inject your library into Lumen 5 framework, so I decided to fork it and made this changes:

• Created corresponding interfaces for Role, Permission, and Rule entities. This change allows replacing default entity classes with external implementations
• Added $entityClass constructor parameters for Doctrine storage implementations
• Added $buildAssignments property to Builder to allow post-deployment updates of roles and permissions
• Moved new $ruleClass in Permission to separate method instantiateRule to make easier injection of Lumen's Container
• Moved new Assignment, new Permission and new Role calls in Builder to separate methods to make easier replacement of default Entity implementations
• Changed private to protected everywhere to make extension of default implementations easier

If you're wondering about having any of this changes in your upstream project, let me know.

BTW, thanks for great project.

[Jalle19]

We use Lumen a lot too so please, make a pull request.

[crisu83]

@AnatolyRugalev you should check our Lumen implementation at https://github.com/digiaonline/lumen-rbac

[Jalle19]

Totally forgot we had that one...