URI based Cache filename

Problem: lengthy filenames, have to cut them after 255 chars, sucks with FB, can cause problems
Pretty-print JSON to cache

Easier for debugging, not really needed though, takes more place for nothing
Create cache file when missing

Damn it...
Store "as it" in JSON (int or str)

The json lib does all the work
Make Accept headers py 2.6 compatible
Add application/xml as html mimetype