Download all YUM packages at once

This is quite a bit faster than invoking YUM for every package needed. Only one mirror query, one dependency resolution...etc.
Best-effort for YUM installation

If we can't get a package or something is wrong, take care of what we CAN install. rosdep will report the problem later, when it finds that the dependency wasn't met.
Merge pull request #214 from smd-ros-devel/master

Install all YUM packages in one command