Thing.clear_cache now only clears the Things cache fixes #48

[sudoaza]

Implemented a fix for #48 with the proposed solution.

Added the option to use the old method since delete_matched may be slow in some implementations.

Also made an exception for Memcache which doesn't support delete_matched.

Maybe it cover #16 too?

[mhgbrown]

https://rubygems.org/gems/cached_resource/versions/5.2.0