Pkgcloud integration

This sets up some initial pkgcloud integration to shunt the images off to a CDN.
Alternate, functional-hipster solution:

On line 19:

```
var twitterCardMaker = function(cdnSslUri) {
  return function (req, res, next) {
    path = req.path();
    // ... rest of twitterCard implementation
  }
}
```

Then, instead of

`server.get('/.*', twitterCard);`

use:

`server.get('/.*', twitterCardMaker(cdnSslUri));`