Feature/cache refactoring

A spot of refactoring in the `com.m3.octoparts.cache` package.

* More meaningful class names
    * `CacheAccessor` -> `Cache`
    * `CacheClient` -> `CacheOps`
* Fixed/added comments
* Added a `RawCache` trait as a facade for Shade's `Memcached`. Before: a `CacheAccessor` wrapping a `Memcached` (which may actually be a completely different cache impl). After: a `Cache` wrapping a `RawCache`.
* Create a `com.m3.octoparts.cache.memcached` package for Memcached-specific implementations