Allow deploys via the API

[SimonRichardson]

This is the MVP for deploying a charm.

There are no resources deployed here. See the example on how you can deploy.

	applicationsAPI := api.NewApplicationsAPI(client)
	if err := applicationsAPI.Deploy("default", "ubuntu", api.DeployArgs{
		Channel:  charm.MakePermissiveChannel("latest", "stable", ""),
		Revision: -1,
		NumUnits: 1,
	}); err != nil {
		log.Fatalf("%+v\n", err)
	}