Express 4

These are the changes which were required to get my application working on Express version 4 with zappa. Creating a pull request, and we can hopefully release this as a beta. 

I wrote a wiki article on moving from version 0.4.x (express 3.x) to version 0.5.x (express 4.x). I'll create a pull request for the wiki as well.

The wiki page for the upgrade is at https://github.com/fgruner/zappajs/wiki/Migrating-from-Version-0.4.x-(express-3.x)-to-Version-0.5.x-(express-4.x)
I don't know how to create a pull request for the wikipage. If someone can tell me that, then I'll create a pull request for that as well.