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)



Yes it is possible to use shapes without the generate method and actually shown in the example. If one wants to combine multiple shapes, then he only calls generate once for the multishape, although there are many child shapes. Same applies to recipe files which inherit most of the multishape functionality.