Use exif.js to extract full exif tags

This change involves the use of the exif.js library (https://github.com/jseidelin/exif-js) to pull out full EXIF data for the photosphere. The rationale for this is that it might be desirable to obtain the latitude and longitude where the photosphere is taken, for example for displaying the photospheres on the map. I also store the "PoseHeadingDegrees" GPano property in the exif object; this has applications including the possibility of linking photospheres to create virtual tours in which the user can navigate from one photosphere to the next, with direction arrows superimposed on the photosphere.