cache: stronger safename; Thanks to Lev Maximov

- safename length limit: 93 chars
- returns str (unicode) on both py2/3
- no idna, just strip all non-ASCII and path-unsafe characters
- the only restriction left: if you pass bytes to safename, they must be valid UTF-8

https://github.com/httplib2/httplib2/issues/27
https://github.com/httplib2/httplib2/pull/74