Rolling buffer for ROSBAG

I needed to have a rolling buffer of messages that I could dump after I saw an error occur.
It seems that having this functionality in vanilla rosbag would be quite useful, so I'm submitting a pull request to discuss it. 
  
Here's something I hacked together to do this. You can start it by running rosbag record --rolling-buffer 30s. 
edit: If you're willing to merge this, I'm willing to clean the code up, and make other small changes, write documentation etc. 

I listen to a topic, and when I get a message on that topic I do a dump. 

@jonbinney @KaijenHsiao