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 :-)
Does Netty have some utility class to print out hexdumps? I only found some code in the ```LoggingHandler``` [1]. I think it's best to move the hexdump functionality into it's own utility class. WDYT?

[1] https://github.com/netty/netty/blob/master/handler/src/main/java/io/netty/handler/logging/LoggingHandler.java#L406