get user by XnUserID

[thiagohersan]

ofxOpenNIUser& ofxOpenNI::getTrackedUser(int index); takes an "index-user-Id", but that's not the same as the XnUserID that's in ofxOpenNIUserEvent.

I've added a ofxOpenNIUser& ofxOpenNI::getUser(XnUserID nID) function that uses a XnUserID to get a tracked user.

There's already a ofxOpenNIHand& ofxOpenNI::getTrackedHand(int index) and a ofxOpenNIHand& ofxOpenNI::getHand(XnUserID nID), so... this is just the ofxOpenNIUser equivalent of that...

Let me know, and I can PR my additions.