cache: include jpg backed storage (~/.cache/darktable/<mip level>/<imgid>.jpg)
cache: some fixes for jpg disk storage backend
cache: fix non-debug compilation
cache: port graphics magick
cache: port lua code to new cache interface
cache: port live view
cache: port facebook
cache: fix flickr
cache: fix picasa
cache: don't write skulls to jpg
cache: also remove disk jpg if thumbnail should be changed.
cache: make jpg removal thread-safe and more efficient

doesn't first write invalid thumb and then delete it, but just delete it.
cache: add functionality to dt-cli to create and store all thumbnails.

syntax:

darktable-cli --generate-cache --core --library /full/path/to/your.db
cache: move jpg backing store to sha1sum hashed directory, too

this way we support multiple --library files again.
cli: fix broken thumbnail strides by using exactly the same rescaling code as the cache internally.
cli: limit created thumbnail to <= mip2 and no skulls
cli: only write new thumbnails (i.e. don't overwrite existing ones)
cache: delete ram->disk serialization code
cache compression: delete it.
cache: put all light table thumbnails into the same hash table