html_entity_decode +fixed cache (doesn't work personally :/)

- Added html_entity_decode on originally ISO encoded Google page to get back the actual UTF-8 characters as it should be. Still don't know why Google doesn't send us UTF-8 directly.
- Effectively grab the thumbnail only if not in cache instead of retrieving it every time.
- Now the cache is based on md5 of the link to avoid risking to get too long file names (256 characters max on some systems).