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)



You are right about the size of that commit ;( Thanks for reviewing it.
I guess I could add a check to the getCloudConstPtr function to see if generate has been called already and call it if not. Question then is how to pass the point density? As argument to the `getCloudConstPtr` call? I do not feel strong about that.