Adds possibility of having empty prefix (at keyword) in controllers

Hi and thank's for a great and flexible library!

Unsure whether or not you would want this in your project, but I needed it in a project of mine so I thought I'd see.

Here is the problem: In my project, someone customised the `matcher` callback to support no prefix (e.g. without @ or ~, etc). This works for the most time very well, but we have some issues with retrieving the controller, and thus the `.atwho('load')` API method won't work.

This is due to the line
```
@controllers[@alias_maps[at] || at || @current_flag]
```

Where as an empty string (`""`) in JS will be falsy and cause `@current_flag` to be used as index instead of `at`. The returning element will be `undefined` instead of the controller at element.

In most cases this won't be a problem, except in our case. But then again, merging this shouldn't have any negative consequences either. As far as I know, there is no issue or "illegality" using an empty string as key in a object literal.