Add Ensenso support

If you want to test please read [the tutorial](https://github.com/VictorLamoine/pcl/blob/ensenso/doc/tutorials/content/ensenso_cameras.rst) first. For the last steps you will, of course, need an Ensenso device!

- The Ensenso grabber has been written like the OpenNI grabber class
- The code contains 2 examples, with/without the Ensenso grabber class
- The code is documented
- Support for Windows has not been tested
- A tutorial has been written to explain how to use the Ensenso in PCL

[pcl-developers topic](http://www.pcl-developers.org/IDS-Imaging-Ensenso-td5709495.html)
If we don't have the macro in `PCLConfig.cmake.in ` then the generated `PCLConfig.cmake` will not be able to check whether the system has the Ensenso SDK or not.

Tthe Ensenso SDK does not install a cmake file (eg: in `/usr/share/cmake-2.8/Modules/`) in the system. Thus PCL cmake file has to embed its own way to find if the Ensenso SDK is there or not.

This is what I understood. Hope it's right.. :sweat_smile: