Add the cache_only configuration option

Hey everyone,

I’ve been trying to get test fixtures working on CarrierWave, and came up with [a solution that works by monkey-patching `CarrierWave::Mount::Mounter#store!`](http://jeffkreeftmeijer.com/2014/using-test-fixtures-with-carrierwave/). That way, you prevent CarrierWave from moving the files from the cache directory to the `store_dir`.

It’d be great to be able to set a configuration option in your tests instead of breaking a method in the mounter, so I added a configuration option called “cache_only”, which does the same thing. What do you think?