Fixes issue where messages are not built before they are used

[ebretl]

Fixes #7 better than just recompiling several times. Gencpp and genpy were not used as components

[ghost]

@ebretl thanks for looking into this, I have been trying to sort this for a while. I tried your changes, but didn't seem to make much difference in the outcome, I still can't compile in the first try. I see you added genpy and gencpp, but those didn't make much difference since they are already dependencies of message_generation. I have no idea what the issue is.

[remod]

I think the best practice is to create a separate _msgs package, and let the driver depend on it. When creating another package which needs these messages (e.g. a user interface), it does not need to depend on the driver itself but only on the message package.

[JWhitleyWork]

@ebretl / @shahvi - See my PR #14. It includes the fix for #7 in the currently-accepted way (gencpp and genpy are deprecated).