added replace claim functionality. fixes #232

 - added ReplaceClaimAsync method to IUserClaimStore interface
 - implemented ReplaceClaimAsync method from IUserClaimStore inside the
   Microsoft.AspNet.Identity.EntityFramework.UserStore<TUser, TRole,
   TContext, TKey>
 - added ReplaceClaimAsync method to UserManager
 - added UserManager tests for ReplaceClaimAsync method.
 - added some UserStore tests for ReplaceClaimAsync implementation of
   IUserClaimStore