Fix InvalidCastException bugs in TryGet- methods

Add a property to get result status in IEither<,> and IReply<,>.
Fix bugs in TryGet- extension methods for IEither and IReply<,>, which
causes InvalidCastException. Rewrite their broken codes.
Merge pull request #9 from takeshik/fix/tryget-methods

Fix InvalidCastException bugs in TryGet- methods