Add support in the SDK for composite commands.

The CLI doesn't support them yet. I don't know of a good way of doing it.
If you consturct the sub_commands using the parse method of the corresponding
classes, there would be extra 'can_notify' and 'label' keys inside them
which the API would ignore. Only the options given directly to the composite
commands are acted upon by the API.
Add support for macros in composite-commands
Merge pull request #46 from mindprince/composite-command

Add support in the SDK for composite commands.