IDE friendly doc block for Cascade_Proxy_PassThroughGateway.

````
  $accessor = Cascade::getAccessor("namespace#data_format");
  /* @var $accessor Cascade_Proxy_PassThroughGateway */
````

Sometimes newbie get confused how Cascade's proxy works. maybe this will help them.
Merge pull request #10 from chobie/improve-passthough-gateway-docblock

IDE friendly doc block for Cascade_Proxy_PassThroughGateway.