Manual payment support through Drupal Commerce order detail page.

Same as https://www.drupal.org/node/1888056 but for Commerce Sagepay Integration. I've had a couple of orders recently that the payment hasn't "Completed" as far as Drupal is concerned, but it has as far as sagepage is concerned.

I'm comfortable with the causes of the errors, 404 error was the cause of 1 of them (Unknown what, but it's only happened once) and a forgotten about debug statement and HTML comments in commerce_email left over from example payments and testing of email creation being others, but I wondered if it would be possible to add in a was of triggering a manual payment (With applicable security protocols such as providing the 404'd URL, or the crypt key provided by Sagepay to ensure it is a processed order) Happy for this to be marked as "Closed works as designed" if needs be as I am unsure if there are issues with potential fraud.