Added order_acknowledgement to Orders class

I basically added a single send_acknowledgement() method to the Orders class.  I copied most of it from the send_fulfillment_data() method and added some tests.  I am using it in "production" and it seems to work, although the "merchant_identifier" param doesn't seem to be set.  :/

NOTE: Most of the changes you see in other files are either:
 - my editor (Sublime Text3) truncating spaces at the end of lines
 - my OCD kicking in and reformatting code so that the '=' line up