Apply resource pattern to error describer service

- Follows Resource pattern (IdentityErrorDescriber.StringName, or FormatStringName(arg1, arg2)
- Also cleaned up optional services, by allowing null in constructor
- SignInStatus -> SignInFailure and introduced SignInResult to make SignInManager APIs consistent with IdentityResult (but no strings needed for SignIn)

Fixes: https://github.com/aspnet/Identity/issues/86, https://github.com/aspnet/Identity/issues/176, https://github.com/aspnet/Identity/issues/287 and https://github.com/aspnet/Identity/issues/177

cc @divega @suhasj 

https://github.com/aspnet/Identity/issues/176 is asking for this, basically if you extend SignInManager and want to return your own specific SignInResult, without a class, they are stuck introducing something custom.  They can return their own AppSignInResult with additional methods this way