Pass URIs through the Cordova resource API.

Hi there,

I found that the latest File plugin doesn't seem to work properly with webintents, due to its custom cdvfile:// URI scheme. This fixes it by delegating the handling of "special" URI schemes to the
relevant plugins, through the mapUri() method in Cordova's resource manager.

This seems to be mostly undocumented (as everything else in Cordova); I cribbed this technique from the file-transfer plugin.