Expose configuration.

This came up while coordinating two different libraries that each wanted
to `RSVP.configure('onerror', someFunction)`. As the tests make clear
the last one "wins" and any prior error functions will be clobbered.

This change allows me to check for the existence of an existing
`onerror` so that my library isn't a bad citizen in the RSVP ecosystem.

Additionally, this change allows libraries to chain `onerror` calls.
This might look like:

```javascript
var priorOnerror = RSVP.config.onerror;

RSVP.configure('onerror', function(error){
  // do something important here
  // then call the prior onerror handler
  priorOnerror(error);
});
```

This also moves the `configure` function into `config.js` (which seems
more appropriate).