Better API utils (wrapper around $.ajax)

The diff says it all. Example:

```js
API.post('/users/'+username+'/documents', {
  token: true,
  client: true,
  data: {
    document: { /*...*/ } // NOTICE: no JSON.stringify here!
  },
  success: function () { /*...*/ },
  error: function () { /*...*/ }
});
```