Fix crash with bad monitor id; add method to get full Oculus pose

[totalgee]

We now figure out the Oculus monitor id in a more reliable way. Previously, it was sometimes crashing in fullscreenOnRift() because hmd->DisplayId was uninitialized.

I also implemented the missing getTranslation() method (was declared in the header but not implemented), and added a new getFullHeadPose() method, that returns the full pose of the Oculus camera (including its parent ofCamera).

This is a better fix than my earlier pull request, which I will retract.

[totalgee]

Here are my comments from the other PR, which I retracted (this PR fixes the same problem, but in a more general way):

I was trying ofxOculusDK2 on with oF 0.9 RC2 (on OS X 10.10.5 with Xcode 7.1), and found the hmd->DisplayId field is not valid (therefore accessing displays[whichDisplay] on the next line crashes). It looks like their DisplayId field may have been scheduled for deletion.