Throwable.getMessage() is not mapped to Exception.Message

[paulirwin]

Java's e.getMessage() converts to e.GetMessage(), which doesn't compile against C#'s Exception; it should map to the Message property. TypeHelper.ReplaceCommonMethodNames (TypeHelper.cs:147) already maps similar cases like hashCode/getClass/close, but that table only rewrites names, so mapping a method call to a property access needs slightly different handling. Noticed while working on #65.