Merge pull request #29 from AlexReimann/indigo

Stage launch use env variables
Add turtlebot_navigation to turtlebot_gazebo depends

gmapping_demo.launch depends on it.
Revert "Stage launch use env variables"
Merge pull request #30 from turtlebot/revert-29-indigo

Revert "Stage launch use env variables"
Merged
Merge branch 'indigo' of https://github.com/AlexReimann/turtlebot_simulator into indigo