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
@HaoK sure, will do that once this's merged (to be able to point to a place).