Better cache dir layout

[dionorgua]

Currently everything is stored in one directory (aka flat layout). At least we should use subdirs to make sure we don't put all files to one directory.

But we can do smarter and also use something more human-readable (like have some subdir names based on actual image URL). Like domain/safe_path/hash

[dionorgua]

I've found that subdirs are already used in latest git. I've just added url file to store original URL.
Not perfect. But it'll allow 'migration' in future if needed.