Obfuscate plain text mails and other major improvements

Obfuscate plain email text (no link) and don't replace text
Don't replace `<a href="mailto:">` tag content anymore
Don't parse non-HTML (CSS, JS, ...) requests anymore
Make obfuscation more robust
Add more tests (including robot)

This refs collective/collective.geotransform#4 and collective/collective.geotransform#8