Added a test case for forthcoming automatic memory-warning handling.
purgeGlobalCaches is now private.
Implemented automatic response to memory warnings.

This would fix the failure of the last relevant test assertion, but another failure (caches don't get warmed after purge) stands in the way.

At any rate, fixes #24.
Re-create the cache if it's been purged.

Fixes the last remaining memory-warning-related test failure. Refs #24.
Moved ivars from interface to implementation. Fixes #28.
Moved this comment up above the other two methods to which it applies.
Unchecking the time-only test cases in both targets, since these won't be fixed until after 0.7. Refs #30.