Created shape generator in simulation module

This pull-request is the second part of my GSOC 2014 project.
- Adds the shape generator to PCL within the simulation module.
- Added simulation to doxygen generation
- Added an example under pcl_example_shape_generator.
- Added two tools, generate_shape and convert2pcd
 - generate_shape uses instructions found in text files to assemble scenes and objects
 - convert2pcd can read stl and obj file and samples on the faces with a given point density and saves a pcd file (unlike obj2pcd it samples on faces rather than on the vertices, allowing constant point density).

![comparison_obj2pcd_convert2pcd](https://cloud.githubusercontent.com/assets/3592251/4207431/3d686a4e-3856-11e4-9a05-9360b3f0cfc3.png)



Do we need the `Shape` suffix in this and the other functions? All of them belong to the shape class, so maybe its superfluous.