Alternate cookie/redirect policy

Hey Carsten! 

A couple of days ago, I forked your extension to implement a new cookie policy. In this version the cookies just serve to prevent an automatic redirection from standard to mobile. They do not force a redirect anymore. The user can navigate back to the mobile version of the page by typing the mobile domain name into the address bar. Additionally the cookie domain is now configurable, so the cookie can span multiple sub domains. 

If you are interested, please feel free to merge. 

Cheers,
Hannes