Allow Admins To Flush Rails Cache

This PR puts a simple clear Rails cache button on the Admin -> General Settings.

This isn't a long term fix for all of the necessary cache busting that needs to happen when models change, but this is hoping to alleviate the tons of requests from API clients, other engineers, E-Com managers, to "why haven't these changes taken effect" i.e. when re-ordering classification position for products and taxons. The end result is some engineer having to log to console to fire off a a ```Rails.cache.flush``` cause "it's important that something on the store changes".

On a side note, I remember Magento having both a clear cache and clear object cache. Hehehe. Good times.

Thoughts welcome....

![screen shot 2014-11-14 at 6 42 11 pm](https://cloud.githubusercontent.com/assets/134368/5055236/7350cbf4-6c2e-11e4-8fb8-275ba81cdbe2.png)

![screen shot 2014-11-14 at 6 42 17 pm](https://cloud.githubusercontent.com/assets/134368/5055237/79645934-6c2e-11e4-99b3-4daa2a1c8aa9.png)