from  Benoît Perrot  <benoit@lrde.epita.fr>

	Add the backtrace command to the shell.
	
	* src/vm/cpu.hh, src/vm/virtual_machine.hh:
	Add an accessor to call stack.
	* src/shell/cmd.hh, src/shell/shell.hh, src/shell/shell.cc:
	Parse and execute backtrace command.