Split a JSON byte stream into JSON objects/arrays. Fixes #2536

Motivation:

See GitHub Issue #2536.

Modifications:

Introduce the class JsonObjectDecoder to split a JSON byte stream
into individual JSON objets/arrays.

Result:

A Netty application can now handle a byte stream where multiple JSON
documents follow eachother as opposed to only a single JSON document
per request.

@trustin @daschl @normanmaurer please review :-)
Insted of adding the frame directly to the output buffer, we need to give a user a chance to filter it, just like we do in `LengthFieldBasedFrameDecoder.extractFrame()`.