Add ability to disable porter for a location

[jaehocho]

I am running Rails and Wordpress under a same vhost. Rails upload works fine with porter, but it breaks upload on Wordpress. Wordpress is not aware of porter, so its upload is broken. If 'Porter Off' can be supported on the location where Wordpress is running, it would fix the issue.

A workaround is to host wordpress on another vhost, but it is not available in my case.