Nodelet for CropBox filter

Hi,
I have created this nodelet for the cropbox filter, it is basically a copy of the [Passthrough nodelet](http://wiki.ros.org/pcl_ros/Filters#PassThrough) using the [CropBox filter](http://docs.pointclouds.org/trunk/classpcl_1_1_crop_box_3_01pcl_1_1_p_c_l_point_cloud2_01_4.html)

I have tested it using pointcloud data from a sensor using a ROS node in a 64bit Ubuntu 12.04 machine.
If there's something wrong, please, let me know.

In case it is accepted I don't mind write the documentation in the wiki.

Thanks!