allow setting custom headers on requests and handling of response.

[nw]

Adds Model.headers(Object) allowing for custom headers to be set on all requests.

Example:

User.headers({
  'X-CSRF-Token': 'some token',
  'X-API-Token': 'api token' 
});

• All model static requests now pass 3 args (error, [objects|model], response) to callback.
• All model method requests pass 2 args (error, response) to callback.
• save & destroy emit self & response to Model listener.

---

This behavior is very helpful when working with 3rd party apis.

Having the ability to easily acquire the response object allows the ability to monitor api limits (ala github & twitter) and hook in caching mechanisms. As well as more robust error handling.

[tj]

perfect! thanks man