rosconsole format for rospy

quick & dirty solution for issue:
https://github.com/ros/ros_comm/issues/517

Since in python it uses [logging module](https://docs.python.org/2/library/logging.html) and in C++ it uses [log4cxx](http://logging.apache.org/log4cxx/index.html), I could not find 1 to 1 translation for some outputs. For example, `${thread}` and `${logger}` does not match well with that of C++.

Also I was not sure if this file was a good place to write this functionality. @dirk-thomas recommended to check out [this file](https://github.com/ros/ros_comm/blob/fab4840ef79f5334bcd043e057a1909183842159/clients/rospy/src/rospy/core.py#L119) but this file was easier to make change. Let me know if you have suggestions,
I find it rather unintuitive to append the time at the end of the string. Would an additional replacement of something like `walltime` make more sense?