Added user mapping to Tokenizer

Fixes #1566

This PR adds an equivalent to the forms authentication `IUserMapper` to allow users to provide a custom `IUserIdentity` implementation.

I'm not sure that `IUserMapper` is the best name though, since this doesn't map a `Guid` to an identity, but rather just creates an identity based on username and claims. 
Any suggestions? `IUserIdentityFactory`? `IUserIdentityProvider`? `IUserIdentityResolver`?
Why internal?