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 I agree within this context. I'll change accordingly. 

> Note: However, I have doubts about claims being not unique. I am feeling that there should be some uniqueness constraint around claims but I'm not sure where they should fit in. Also, that's outside the scope of this PR I think :smile: I'll probably open up another issue to discuss that later.