uses else clause
Merge pull request #402 from efernandez/try_except_else

uses else clause
add topic/connection statistics

This patch adds a module to roscpp/rospy that monitors some metrics
over every ROS connection (period, dropped messages, traffic, age)
and periodically sends out aggregated statistics data over a common
topic /statistics. It is designed to be lightweight so it can be used
in "production" environments.

Statistics monitoring is disabled by default. It has to be enabled
explicitly using "rosparam set enable_statistics true".
Merge pull request #398 from ros/statistics

Topic/Connection statistic measurements
the latest change requires the upcoming version of rosgraph_msgs (#398)
allows for non-scalar output types
adds --import arg
supports String by checking only for list or tuple, instead of Iterable