Convert rndf_gazebo_plugin from system plugin to world plugin

[Javier Iván Choclin]

We are using a system plugin, we want to use a world plugin so everything is specified in the SDF.

[Agustin Alba Chicar]

As for the record, we talked with JChoclin about the camera auto position. This feature must be removed as we moving one step forward in the abstraction layers and we don't have reference to it.

[Javier Choclin]

Yes, that feature will be present, but it should be declared on the world file, there is specifics tags that handle the camera.

[Javier Iván Choclin]

Working from World plugin. Camera movement is not working. refs #47

→ <<cset 6b17f67d2805>>

[Javier Iván Choclin]

Removed the camera position, also refactored code. ref #47

→ <<cset 03b95680406d>>

[Javier Iván Choclin]

Modify all the samples to work with the plugin. refs #47

→ <<cset 474fab8ca73f>>

[Javier Iván Choclin]

Reduce the world definitions so the include models instead of defining them. refs #47

→ <<cset f961ea800e9d>>

[Javier Iván Choclin]

Updated the wiki. refs #47

→ <<cset 614387dabd06>>

[Javier Iván Choclin]

Changed the namespace. refs #47

→ <<cset d7a6a2dc51c0>>

[Javier Iván Choclin]

Changed the world files to use the gazebo plugin path variables. refs #47

→ <<cset 3e736b3091fa>>