This PR adds the ability to remove elements from a cache without calling any configured eviction callback.

My use-case is I have a cache of non-trivial io.ReadCloser implementations and the eviction callback calls Close() on any "expired" entries. I want to be able to retrieve a cache entry and remove it from the cache to prevent another goroutine using it, however currently doing this calls Close() which then renders the io.ReadCloser unusable.

Fixes #171