Allow elbadmin add/rm commands to use a list of instances instead of a single instance

This pull request allows to use the elbadmin command line tool, with multiple instance ids for the add and remove|rm subcommands. For example : 

    elbadmin add my-elb i-42f2ea21 i-42424242

will add instances i-42f2ea21 and i-42424242 to the "my-elb" ELB in a single command, and thus a single API call.