Add basic support for modes: none, new_episodes, once

We also needed support for the `none` mode, so I created a very simple implementation. 

Please let me know if you have any feedback or if you would like me to create some automated tests. I wanted to check to see if the implementation was acceptable first.
I added this property because of some strangeness with multicurl. I was experiencing issues where the Cassette would be new but the Storage was "old" and this seemed to be the best fix.