speed up message_generation() macro

This patch implements the generate_messages() macro using just cmake. Removing the python calls provides a significant performance increase. From my tests I've found it to be ~0.2s faster on average. In our project we have ~30 packages that generate messages or services and it saved ~8 seconds off our cmake configure step.

Also, I went with option 1 for #41 as we felt the build should be automatic to and incurring a reconfigure in our project is a small time cost.