[gazebo_ros] Fix for #246

Fixing issue #246 in gzserver.
Merge branch 'indigo-devel' into patch-1
Extended the fix for #246 also to debug, gazebo, gzclient and perf scripts.
Merge pull request #247 from peci1/patch-1

[gazebo_ros] Fix for #246
move declaration for DefaultRobotHWSim to header file
adding separate new gazebo_plugin supporting hw_interface switching