Feature/support multiple activation and deactivation

[HannesSommer]

This supports linking multiple REGISTER_ROSCONSOLE_BRIDGE; together and therefore enables ROS libraries to take care of this for non-ROS libraries they might depend on without risk.

This is one possible first step to improve on issue #12 .

The extra complexity to support deactivation is not necessary.
Currently it is just used to allow the "cleanup" test to remain partial functional. Admittedly this way it does not serve as much of a purpose as before. The problem is the introduced singleton that takes care of disabling the output handler at program termination. I don't know an easy way to test this from within the program without extra complexity in the library itself. However, the standard guarantees the destruction of static locals within the program termination phase - as far as I know. And the deactivate functionality is still successfully tested:

[ WARN] [1501317381.498355918]: This warning should be served through rosconsole
Warning: This warning should be served through the original console_bridge output handler
at line 9 in .../rosconsole_bridge/test/cleanup.cpp

(its output on my system)