[Invite Users feature restricted for managing users]

 - Invite Users can accessed only by manager users restriction added.
 - Spec request feature added.
[Invite Users feature restricted for managing users]

 - check privileges of manager and managing users modified in users invitations controller.
 - spec request renamed.
Merge pull request #30 from insignia4u/invite-users-feature-restricted

[Invite Users feature restricted for managing users]