Serialization support

### Example 
```js
var cache = LRU( {max: 500, maxAge: 60000})
  , otherCache = LRU(50) 

cache.set("key", "value");
anotherCache.load(JSON.parse(JSON.stringify(cache)));
```

### API additions 
* `toJSON`: Returns a ready to be serialized array
* `load`: loads a serialized array into the current cache instantiation. Method name is the @mikeal proposal in #18

### Backwards compatibility
`true`

### Rationale
The approach taken to serialize/unload caches is KISS. Just exports the entries (`key`, `value` and `now`) and adds it to a new cache one by one. Benefits:
 * The receiver cache options (`max`, `maxAge`, `length`...) can be completely different. 
 * Cloning caches is now possible. Using `JSON.stringify` +  `JSON.parse` causes a deep copy clone
 * Not exporting internal counters (`_lru`, `_mru`...) safeguards dumps against internal refactors in future versions

closes #18