OpenNI2 driver modification including device serial number.

Hi  EduFdez,

I really apologize for my late pull request.
This pull request includes my first request and a little modification of the 'openDevicesBySerialNum' method.
I changed the argument of this method from std::vector to std::set, because std::set can ensure that it's elements are unique.
And I modified the method's implementation to integrate with my programs.

If that interferes with your new programs, I will edit the header file to recover the declaration of 'openDevicesBySerialNum'.

You can execute code formatting to normalize indent or line spacing and can remove my comments freely.

Thank you.