API: Prevent large images from repeatedly crashing PHP on resize

Add unit tests, fix incorrect class instanciation

Prevent image manipulation methods from causing repeat GD failures

Use destructor to handle lock file removal

Remove attempts to call parent destructor - there isn't one

First draft of using SS_Cache API

Updated unit tests, minor fixes/tweaks

Updated onBeforeDelete logic and unit tests

Add dev task for clearing cache of image manipulations

Store manipulation method/arguments in more accessible way in cache

API: Prevent large images from repeatedly crashing PHP on resize