Response handler

Instead of having response handlers like this in our code:

```
+      if ( ! $poll ) {
+        header( "HTTP/1.0 500 Internal Server Error" );
+        echo json_encode( array( 'success' => false, 'message' => __( 'There was an unexpected error when creating the poll', 'WPPP' ) ) );
+        exit;
+      }
```

It should be an API provided in hm-rewrite.

From https://github.com/tcrsavage/wp-pretty-polls/commit/a1bf15d25851f0bd95df1bec7264c6a3231ad183#L3R12