Caching helpers for Rails

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Based on cached_model by <a href="http://dev.robotcoop.com/">robotcoop</a>. We started out with their code, but added so much that it merited its own plugin. Apologies for not doing proper contributions to their project instead of building our own, but we are limited by commercial requirements rather than open source idealism. === Cached Super Model: a short introduction. This works a lot like <tt>acts_as_cached_model</tt>, but with a few subtle differences.