ros_bag 508

Reworked bag.py to provide various methods for getting info about the bag as a programmatic object rather than in string or yaml format.
PEP8 (http://legacy.python.org/dev/peps/pep-0008/) says to put whitespace around operators, i.e. `num_messages += topic.message_cout`. It might not be like this all throughout the file, but given the choice on new work you should prefer this style.