(TWILL-45) Included Throwable information in LogEntry

1. API change in LogEntry class
  - Add getThrowable() method to return LogThrowable
  - Deprecated getStackTraces(). Implementation is to lookup stack trace from the LogThrowable.
2. Refactor Gson encode/decode of Log entry related classes to simplify logic.

Signed-off-by: Terence Yim <terence@continuuity.com>