rtt_rosclock: Adding SimClockActivity

This PR expands rtt_rosclock to incorporate a fork of @meyerj's rtt_gazebo GazeboActivity with some modifications that enable it to use either the ROS `/clock` topic or a manual clock source if there is one in-process. See the [rtt_rosclock/README.md](rtt_rosclock/README.md) for example usage.

This PR also incorporates changes included in https://github.com/orocos/rtt_ros_integration/pull/26