Scope the List query in the KV Enumerator

Closes #58 

This allow you do do things like:

```ruby
app[:items].start(:bar).before(:foo).map {|kv| [kv.key, kv.reftime] }
```

And it will enumerate over all keys in that range, following paginated results as it goes.

@roder @jsmecham any thoughts on whether I should set a default for the page size, to change it from 10 to 100 and do fewer requests?