Logging changes.

1. Anything that comes in from OSG will always be logged; it seems
somewhat redundant to have to set the FG log level to be identical to
the OSG log level to see these messages.

2. Console output will now have delta time (since start), priority and class.

3. Log file output wil also have delta time (since start), priority and class.

Times are displayed as delta decimal seconds since the logger was created

e.g.
  1294.06 [INFO]:terrain    Some log message

simgear/debug/logstream.hxx

@@ -52,6 +52,7 @@ protected:
 	bool shouldLog(sgDebugClass c, sgDebugPriority p) const;
 
     static const char* debugClassToString(sgDebugClass c);
+    static const char* debugPriorityToString(sgDebugPriority p);
 private:
 	sgDebugClass m_class;
 	sgDebugPriority m_priority;