add onForRuntime() method to jsdIDebuggerService to let native code turn on the deubgger (on() can only be called from js)
register an app-start observer so we can turn on the debugger at startup if "js.debugger.autostart" pref is true.
r=peterv, bug 81840
move from pc as a ulong to pc as an object wrapped around a uword
relocate jsdService constructor to jsd_xpp.cpp in order to initialize the global
service