added replace claim functionality. fixes #232

 - added UpdateClaimAsync method to IUserClaimStore interface
 - implemented UpdateClaimAsync method from IUserClaimStore inside the
   Microsoft.AspNet.Identity.EntityFramework.UserStore<TUser, TRole,
   TContext, TKey>
 - added UpdateClaimAsync method to UserManager
 - added UserManager tests for UpdateClaimAsync method.
 - added some UserStore tests for UpdateClaimAsync implementation of
   IUserClaimStore
I'm also not sure whether the EF store should be updating the existing POCO or deleting it and creating a new one, but that's an implementation detail for the Store I guess, but this does more closely match an update semantic you originally intended.