New methods to get multiple entities by ids

Suggested methods could be:

```
IList<T> GetMultiple(params TKey[] ids);
IList<T> GetMultiple(IList<TKey> ids);

// not sure the best name here
IDicitonary<TKey, T> GetMultipleDicitonary(params TKey[] ids);
IDicitonary<TKey, T> GetMultipleDicitonary(IList<TKey> ids);
```

Need to decide whether this does multiple calls to Get or build a query so it's done in one call to the data store.
