Add HTTP headers to RESTAdapter ajax calls

Some APIs require additional HTTP headers, for example an API_KEY so have added the ability for this.

Followed the same pattern as used for `namespace` by checking for an `apiHeaders` property on the adapter. 

Example configuration.

```javascript
DS.RESTAdapter.reopen({ 
  apiHeaders: [[ "API_KEY", "secret key"], ["ANOTHER_HEADER", "asdsada"]]
});
```

I couldn't find tests for constructing the ajax hash so was unsure where to start. Would appreciate any guidance.