add warning when queue_size is omitted for rospy publisher

The warning can be suppressed by setting:

    export PYTHONWARNINGS="ignore::SyntaxWarning"

I choose `SyntaxWarning` since I wanted to make it distinguishable from the existing warnings in rospy (which are by default `UserWarnings`).

This **will** lead to a lot of warning for every existing Python code out there. I am just saying this explicitly. We should consider that and also might want to ask the community about that?

@esteve @tfoote @wjwwood Please review and provide feedback.