Add "default" handlers for commands/requests

##### Resolves #82

Adds special keyword `default` to Requests and Commands so that when a request/command handler isn't registered it will be called instead.

In order to shave off some bytes I added aliases:
```js
// Commands#command
var commands = this._commands;
// Requests#request
var requests = this._requests;
```
This test isn't doing anything?