camera plugin: Added first implementation of cameraPlugin.

  It works, the flickering of the image has been solved.

TODO gazebo crashes while closing ... as usual :) to be fixed
[travis] Added libavcodev-dev installation when using Gazebo 3.0
[travis] fix syntax error
[travis] fix typo
[travis] added libavformat-dev if using gazebo 3.0
[travis] added libswscale-dev if using gazebo 3.0
GazeboCamera: removed debug stuff and unused code. Uses correct disconnect
event function
Merge branch 'cameraPlugin': new plugin for cameras is available.
first release, camera size of fixed to 320x240, image type rgb.

It works.