Feature/yaml integration test generation

Fixes #461  

This PR includes a generator for the official integration test suite found here:

https://github.com/elasticsearch/elasticsearch/tree/0.90/rest-api-spec/test

These tests are generated under the `Nest.Tests.Integration.Yaml` project. Be careful when running these tests though as they will clear your entire elasticsearch cluster on localhost:9200 prior to each test (as required by the official spec).