custom cache engine support

I'm using hapi 1.7 and attempting to use a custom cache engine.

``` js
var customEngine = new CustomEngine({});

Hapi.createServer(8080, {
  cache : {
    engine : customEngine
  }
});
```

I get a validation error "the value of cache must be a string". It appears that its only possible to use one of the default cache engines (memory, monog, redis).
