Add support for inline images

Author: chandon

lib/SparkPost/Transmission.php

@@ -30,6 +30,7 @@ class Transmission extends APIResource {
     'recipients'=>'recipients',
     'recipientList'=>'recipients.list_id',
     'attachments'=>'content.attachments',
+    'inlineImages'=>'content.inline_images',
     'template'=>'content.template_id',
     'trackOpens'=>'options.open_tracking',
     'trackClicks'=>'options.click_tracking',