Allow displaying images and video in hotspots

Hello there :)

As previously discussed, I've added some capabilities to hotspots:
* Displaying images on hovering
* Displaying videos on hovering, using the standard html5 <video> tag.

As a side effect, as I needed some icons to represent those in hotspots, I fiddled in the css to make it play a bit nicely (but not perfectly) with font-awesome.

As this is my first commit to pannellum, please feel free to correct whatever could be wrong, or to point me in the right direction... Thanks :)

Guillaume