added functionality to automatically_authenticate

Hey Paul,

I just added a fbconsole.automatically_authenticate that addresses the issue I raised earlier this morning. It includes documentation and the addition of [mechanize](http://wwwsearch.sourceforge.net/mechanize/) to the install requirements. To be honest, I'm not sure how nicely this plays with python 3 but I had some code written elsewhere that I was able to easily rip so I thought I'd post it as a candidate solution. 

Happy to help modify this as necessary...

Cheers.
Dean