fix for lp:1214209: Allow add-machine to take a -n param for adding multiple machines in a single command
don't allow -n when specifying a placement directive
better output for add-machine -n
more consistent error output for single and multiple adds
Merge pull request #198 from mattyw/add-n-machines

Allow add-machine to take a -n param

Fix for https://bugs.launchpad.net/juju-core/+bug/1214209

Allow add-machine to take a -n param to add multiple machines