Issue #75: wired CacheLoader where applicable.

I can create a separate commit for this, but I do believe these changes can be in this same pull request. The task here was to wire and fix usage of `CacheLoader` API. As part of the task, I went ahead and fixed the documentation either; because both `CacheWriterException` and `CacheLoaderException` are unchecked, adding them to the interface definition serves mostly a documentation purpose.