Performance improvements: caching HTML and skipping change checks on images.
Also refactored file return and cache directory creation.

Temporarily added timing logging.  Still to do: make this optional, fix broken slideshows
Changed URLs of external resources to allow HTTPS access
Moved all resources (CSS, Javascript, images) into a single directory.
Updated references to it.
Fix bug with folders with no geotags.
Still generate .points.xml even if no images have geotags.
Force filetype for resized THM files to be JPEG.
Cached picture pages now include width and slideshow time.
Return HTTP 204 correctly for background images of folders with no images.
Video thumbnails in album view are correct size.