Improve create index settings

This allows you to specify the `refresh_interval` and custom settings during index creation:

```scala
client.execute {
  create index "places" shards 5 replicas 1 refreshInterval "10s" indexSetting("compound_on_flush", false) mappings (/* mappings... */)
}
```

I've also tried to clean up the create index docs and split them into an intro section and a more in depth reference section.