Added endpoint for exporting templates

@kmcphillips 

Instead of playing around with the `index` action I just added a new one. For some reason `index` responds to `html` and `xml` and I didn't want to break anything by skipping the `shopify_session` around filter for `index`.

`export` authenticates using http basic auth since `authenticate_or_request_with_http_token` was only introduced in Rails 3. Returns a 403 if auth fails or if no shop is found.