Add an interactive shell.

        * src/vm/cpu.hh, src/vm/cpu.cc:
        Add a print operator.
        * src/vm/virtual_machine.hh:
        Add breakpoint suport and some shell entry points for step by step
        execution.

        * src/shell/cmd.hh,
        * src/shell/shell.cc, src/shell/shell.hh:
        Implement interactive shell, using readline.
        * src/shell/shell-tasks.cc, src/shell/shell-tasks.hh:
        Add shell's tasks.
        * src/shell/Makefile.am:
        Distribute shell.
        * src/modules.hh:
        Declare shell.

        * configure.ac:
        Check for readline library. Add shell directory.
        * src/Makefile.am:
        Add shell directory and link with readline.