Add support to override method inside ajax posts

If you can't use `PUT`, `DELETE` etc. on your webserver the is method overrides by drf.

http://django-rest-framework.org/topics/browser-enhancements.html

but just if you send the forms with.. yeah. a form ;)

but as soon as you want to use an ajax call like this:

```javascript
var form_data = form.serializeObject();
form_data['_method'] = form.attr('method');
$.ajax({
     type: 'POST',
     data: JSON.stringify(form_data),
     contentType: "application/json",
     ...
```
it don't work. the current method overrides are only available for pure form. cause of code like here:
https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/request.py#L277

test cases + docs still missing. coming after review. 
would be nice if this comes in. :cake: