Add option to cache sickbeard images

[N3MIS15]

This gives the user the option to cut down on requests to the sickbeard api. Every show makes a request for an image.
While this seems like a no brainer (at least to me). It can take a while to initially download all the images if you have a large library. That is why i have made it optional.

The naming convention I chose coincides with sickbeards, so its possible just to copy sickbeards cache over.

FYI it took about 3-4 minutes to cache 235 images on my test.
Tested on windows only.

EDIT:
If you feel this would be useful i will look into doing the same for couchpotato and headphones

[titan04]

I think it will be useful for all library type items. Since banners etc do change as new ones are available or your update/change some of your library, it should scan for changes from time to time to do an update on just changed files.