Make ExceptionLayoutRenderer more extensible

Right now there's no easy way to tweak how a particular layout token (i.e. Message or Data) is rendered by extending ExceptionLayoutRenderer, because the Append... methods are private static
- Convert Append... methods from private static to protected virtual
- Change ParseMethodNameFromStackTrace from private to protected
Fix compile issues/warnings
Merge pull request #430 from SurajGupta/master

Make ExceptionLayoutRenderer more extensible