Add ability to change pov with mouse without interfering with headtrackr

Hello Alex,

In the past year, I've also myself been playing with headtrackr ( http://dev.topheman.com/parallax-js-with-headtracking-support/ ) and streetview ( http://dev.topheman.com/topheman-street-view/ ), this friday, I passed by the headtrackr repo of @auduno and ended on your demo ... I was having the same idea (but with something in 3d in BabylonJS).

When I tried your demo I felt something was missing, I couldn't change the point of view ... So I coded the feature on top of your app. I didn't touch any part of your code that didn't need to be touch, don't worry.

If you want to test before accepting the PR, it's here : http://topheman.github.io/street-facing/

Tophe

PS : I fixed a bug with the headings (you can now face any angle) - I didn't mention it in the releases notes.