Removing ROS

As per our [pcl-developers] discussion, this (214 file) commit aims to completely eliminate ROS clashes. It does so by:

- Renaming std_msgs and sensor_msgs to pcl_std_msgs and pcl_sensor_msgs, and changing the directory structure to reflect this
- Prefixing all clashing types (Header, Image, PointCloud2, PointField) with PCL, and changing the header files to reflect this
- Updating the API / Documentation accordingly
- Getting rid of all references to USE_ROS, find_package(ROS ...), and add_ros_links references

I tried my best to make sure I fixed everything without any annoying casualties (e.g., openni_wrapper::Image, comments pertaining to Header files or Image Processing, etc). Let me know if you catch any.