Make it easier for clients to use badgehost programmatically

This PR makes it possible for clients that install badgehost via npm to easily use it in test suites and such. It does this by providing defaults for `staticDir` and `assertionDir` in the app constructor, and by adding an `app.url()` method which returns absolute URLs to badgehost resources. For example, with this PR, the following code snippet works:

```javascript
var app = require('./').app.build();

app.listen(function() {
  console.log("your badge is at", app.url('demo.json', {set: {lol: 1}}));
  // the above produces something like "your badge is at http://127.0.0.1:52354/demo.json?set=..."
  this.close();
});
```