Adds a number of improvements and bugfixes to the JSON and utility classes:

JSON framework:

* A new `BufferingJsonpMapper` allows the creation of `JsonGenerator` that store JSON events in a buffer that can be replayed. This is useful to efficiently create synthetic JSON documents.
* An additional `JsonpMapper.deserialize` method variant accepts the current JSON event. Fixes #741
* The Jackson implementation of `JsonpMapper` now enables the `ACCEPT_SINGLE_VALUE_AS_ARRAY` deserialization feature. This allows single values in a JSON stream to be considered as a single-value collection.

API & transport framework:

* `ElasticsearchException` now holds the low level http response, so that the application can inspect the details of the error.
* Endpoints now have a `call` methods, to make calling endpoints programmatically easier. This is for advanced use, as an application will normally use the `ElasticsearchClient` that hides endpoint objects.
* A `BinaryDataResponse` can now easily be created from a byte array.

Test framework:

* The `ElasticsearchTestServer` now tries to contact an Elasticsearch server running on `http://elastic:changeme@localhost:9200` before spawning a container.
* A `MockHttpClient` has been added that allows writing integration-like tests without requiring a running server. This is an alternative to using `com.sun.net.httpserver.HttpServer` when we want to test http response decoding but don't need to test the network layer.