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)
I am still not sure what you want to achieve with this. If the comment is correct, then what you need is:

```cpp
camera_ = &( (*root_)[itmCameras][itmBySerialNo][device]);
```

Then `camera_` will just point to that device and it will be easier to access it.

(You'll need to get rid of allocation and deletion of memory for `camera_` in ctor and dtor.)