Update Express to v3 + minor enhancements

Main update is about migrating to express v3.
Please take a more detailed look at 07d88d3 which might not do exactly the same as before.

Additionally, this update 
* allows developing on Cloud9 through added `host` parameter
* adds client side form validation of email input
* marks all required input fields with the specific html5 attribute
* passes the `host` to the socket.io connection (protocol is still missing)

Next step might be to migrate to express v4.