Added ability for applications to use rosbag recorder class and stop recording during usage

The recorder class by default can only stop recording when the node is shutdown. This change adds a flag that allows ROS applications to start and stop recordings without having to deal with implementing their own subscribers and message handling logic.

The only change to the API is a single additional argument in a existing function, and it has a default value. Still, this probably should be targeted for indigo. I'm more just submitting this pull request for initial feedback. Maybe my implementation sucks, please let me know.

This change can be used with, for example, [rosbag_record_cpp](https://github.com/davetcoleman/rosbag_record_cpp/blob/master/src/rosbag_record.cpp)