Merge pull request #681 from dgossow/hydro-devel

re-added media_export
read topic types from plugin xml
removed getTopicTypes / checkTopic from Display interface
added message type metainfo to plugin_description.xml
Merge branch 'hydro-devel' of github.com:dgossow/rviz into hydro-devel
Merge branch 'hydro-devel' of github.com:ros-visualization/rviz into hydro-devel