Implement IRoleStore

[g0t4]

I'll add storage for this in a separate collection. It's just a list of names unless someone wants to extend the model in their own application.

[g0t4]

FYI on the implementation (this is from the summary of RoleStore):

Note: Deleting and updating do not modify the roles stored on a user document. If you desire this dynamic capability, override the appropriate operations on RoleStore as desired for your application. For example you could perform a document modification on the users collection before a delete or a rename.