Added app.baseURL and app.url() method.

This intercepts the calling of app.listen() to remember what port the
app is listening on, and setting app.baseURL to include this value.
app.url() is like app.path() but provides absolute URLs to resources,
which is means that programs importing badgehost can very quickly set up
and use a badgehost instance, e.g.:

```
var app = require('./').app.build();

app.listen(function() {
  var url = app.url('demo.json', {set: {stuff: 1}});
  console.log("get your badge at", url);
});
```