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
So claims are not necessarily unique, I'm assuming the correct behavior is actually to replace all instances of claims with the matching type/value instead of just the first.  Do you agree?  Basically just iterate and remove all matching claims instead of FirstOrDefault..