Added auth_token to mws

Added auth_token to mws __init__. auth_token will be used on all
requests to amazon. Added to the end of the parameter list for backwards
compatibility.

Starting Aug 18, 2014 AMS asks for a MWSAuthToken parameter for all
requests from 3rd parties.  This is optional for 3rd parties until March
31, 2015, when it will be required.  More information can be found here:

http://docs.developer.amazonservices.com/en_US/auth_token/index.html