Added auto sub domain feature for cookies

[rbjarnason]

We run one node.js server that serves multiple domains and we wanted to have the cookies shared across subdomains. The usual way to do this is to set domain: '.domain.com' in the options but this is not possible as we are serving multiple domains hence this simple addition.

[dougwilson]

Thanks for the pull request! On top of the comments I made inline, you also need to add documentation to the README for this feature as well as add unit tests that cover all new branches in code you've created here.