Convert the run command to use the StackFrame system

Convert the run command to use the StackFrame system

* Source/GDBpConnection.m+h:
(run): Now returns a StackFrame object
(processData:): Return nil if the given data is nil
(createStackFrame): If we get an empty NSXMLDocument for the stack, return nil
(updateStatus): If the status is nil, the debugger is stopped
* Source/DebuggerController.m:
(run): Use the stack frame
* Source/SocketWrapper.m:
(receive): If the call to recv() returns -1 (error), return nil string