Implemented the completion suggester for auto-complete functionality

[gmarz]

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-suggesters-completion.html

As the documentation states, this is currently an experimental feature of Elasticsearch, but it works pretty well and it completes the suggestion offering of NEST (as we already have the Term and Phrase suggesters).

[Mpdreamz]

Awesome++ this is a pretty big feature PR. Many many thanks @gmarz looking forward to pulling it in 👍

[gmarz]

Great. There are a few TODOs, namely:

• Fuzzy querying
• Attribute-based mapping
• Unit and integration tests

I'll get around to completing those, but I think this is a pretty functional first pass.

[Mpdreamz]

thanks for the update, you can leave out the attribute based mapping, it will be phased out and only used for the quick wins if you will, forcing a property name or declaring which property is the Is.