Fix for #580

As requested in #580, adds support for per-point alpha in markers. Will show a warning if all alphas are set to zero.

Basically, this is not an API breakage since the documentation on http://www.ros.org/wiki/rviz/DisplayTypes/Marker doesn't say that alpha values are ignored, so they shouldn't. This might break some code though, if someone publishes list markers without setting the alpha value.