.getCachedFileUrl() method fixes #50

This does two things.  First, it fixes a regression from .70 where IS_MOCK is still tested for isCordova() cordova.

More importantly, it adds the 'low level' but very convenient function getCachedFileURL for easy convenience to the direct file url -- this is for contexts that are either divergent or more complex than the methods for replacing the IMG or background-image values.