Added CSRF support.

I don't know if this is what you had in mind when it comes to CSRF support, but this solution works well for me. You simply specify "csrf: 'sometoken'" in Options, like this:
```js
var client = new $.RestClient('/api/v1/', {
	csrf: '{{ csrftoken }}'
});
```
And the token is sent along with all requests using the X-CSRFToken header.

Feel free to pull, or ignore if you have plans for a different approach.