Add a new context processor to determine if a UA is mobile

Hello Greg, 

First of all, thank you for taking time to make this project because it simplifies a lot things.

Let's get started.

What I propose is to add a new context processor that indicates if the request is made by a mobile user agent. Right now there is no way to know this from a template.

This makes sense in case we want to show a way to get back to mobile flavour once we have selected the full one.

Again, thank you for your time and work. Let me know what you think.

Cheers.

Javi