Introduce Group Permissions

[enjaku4]

• Introduce role groups to allow the assignment of multiple roles to a user simultaneously.
• Enable permission checks for an entire group.
• Implement a convenient interface for creating groups and assigning/removing roles to/from them.

[enjaku4]

Too much effort for something that could easily be implemented on the gem user side. As a group is just a collection of roles, such a collection can be defined anywhere in the project and then passed as roles argument to the grant_access method.