Honour the processData flag in RESTAdapter#ajaxOptions.

This change allows users to pass the `processData` flag to the `RESTAdapter#ajaxOptions`. This flag ultimately tells jQuery that it shouldn't perform any transformations to the data passed to the ajax request.

Previously, ember-data didn't honour this flag and would blindly `JSON.stringify` any data passed along for the request. This is normally the exact behaviour you want, but it prevents passing strings or instances of `FormData`, which is useful for file uploads and custom data you want to pass to the server.

Now, if `processData: false` is passed, then ember-data will not perform any transformations to the data object passed. It will also not assume that the content-type is `application/json`.

Would you mind adding a default to true? And documenting the option somewhere?