Implement save and load methods.

Added do_save() and do_load() methods to shell().

do_save() saves the current contact_handler() instance.
do_load() loads the sepicified contact_handler() instance, its a name as
an argument.

Fixed a typo in line 28 calling self.group_list[group].__del__ instead
of self.group_list[group].__del__().

Try to load 'default.sav' if it exists otherwise init a new
contact_handler() instance as default.

Now requires the pickle module.