Feature: sync cache

[vadymmarkov]

Cache is born to be async, so I've decided that there is no place for sync functions in CacheAware protocol, it kinda breaks the whole architecture of the library. But if for some reason you really need to perform cache operations synchronously (on your own risk 😄), there are helpers for that:

SyncHybridCache for HybridCache

let cache = HybridCache(name: "Mix")
let syncCache = SyncHybridCache(cache)

// Add UIImage to cache synchronously
syncCache.add("image", object: UIImage(named: "image.png"))

// Retrieve image from cache synchronously
let image: UIImage? = syncCache.object("image")

// Remove an object from the cache
syncCache.remove("image")

// Clean the cache
syncCache.clear()

HybridCache for type safe Cache

let cache = Cache<UIImage>(name: "ImageCache")
let syncCache = SyncCache(cache)

syncCache.add("image", object: UIImage(named: "image.png"))
let image = syncCache.object("image")
syncCache.remove("image")
syncCache.clear()

This PR also adds source code documentation and this is nice 📖

Fixes #33

[vadymmarkov]

Travis is as always 😬

[zenangst]

I think it should read;

Cache was born to be async.

[zenangst]

Travis...

[arden]

great