Zoom and Pan in Fullscreen

I added zooming and panning in fullscreen mode.  With a touch screen device you can pinch and drag, and on the desktop you can use the +/- keys and drag with the mouse.

Most changes were to content_gallery.js.  I added events for the keys and hammerjs events for pinching, and I added x coordinates to some of the functions for panning.  I also moved the code that updates the gallery size and that updates the active slide CSS to their own functions because I found myself reusing those bits.

My main concerns are consistency on mobile Chrome, which has problems receiving the hammerjs events sometimes.  There is also an existing timeout in the loadImage function that causes a delay in resetting the image out of fullscreen mode.  I'd like to remove this or replace it with a jQuery load event, but I'm not sure if it's there to fix another bug I might uncover.

Hope this adds to the project!