Partial support for latched publisher support

This is a first pass to support publishing of latched topics (see #35). This PR goes hand in hand with its corresponding PR to roslibjs.

This implementation simply passes through a 'latch' param down to the rospy Publisher object.

Since rosbridge might aggregate several ROSLIB.Topic instances to the same rospy Publisher object, proper implementation would require more logic.
In this version, collision situations such as the ones described above are detected and a warning printed to console.

I propose that having this partial implementation will allow to cover more cases than no implementation at all (current state).