initial commit
  "Order" could be used now
removed some dependency that was copied over, in hope of reducing load time. so far no noticable differences
fixed problem for wrong constant names in WeixinMerchant::Model::Base
added User model. it turns out this is needed to make the service API more consistent
added WeixinMerchant::Service implementation. right now it has "get_user" and "get_order" implemented
removed dependency of weixin_authorize gem. the weinxin_merchant service now needs such a client:
  * :is_valid? (so it has to be initlialized before)
  * implements ":user" and ":get_order" (with respective parameters)
The service is also changed from a Module object (singleton) to regular objects, so different service objects could be used for different Weixin public accounts, although at this time only Weixin Service Accouts (微信服务号) could have the "merchant" capabilities.