Providing error on all HTTP error status codes.

Based on https://github.com/adunkman/node-trello/pull/12

When the API returns an error (anything in HTTP 4xx and 5xx range) it's passed back to the user as an error.

An example is

````javascript
var Trello = require("node-trello");
var t = new Trello("intentionally-wrong", "intentionally-wrong");

t.get("/1/boards/intentionally-wrong/cards", function (err, data) {
  if (err) {
    console.log('status:', err.statusCode);
    console.log('message:', err.statusMessage);
    console.log('body:', err.responseBody);
    throw err;
  }
  console.log('success', data);
});
````

The output is

````bash
status: 401
message: Unauthorized
body: invalid key


Error: invalid key
````
The only recommendation I have is maybe something like

``
var statusCode = Number(response.statusCode);
if(statusCode >= 400  ...)
  err.statusCode = statusCode;
```
Just to make sure we're always dealing with a number. `parseInt()` would work as well