Add MsgPack feed unpacker codec

[moshen]

Re-creation of elastic/logstash#1774 per the changes in logstash 1.5.0

[suyograo]

Can you please add documentation (using comments) on what this codec does in asciidoc? This will be converted to html and used as reference docs. Check this for an asciidoc guide: https://github.com/elastic/docs/blob/master/README.asciidoc#asciidoc-guide

[moshen]

Updated. Will squash if desired.

[ghost]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run; then say 'jenkins, test it'.

[kares]

Nice PR, seems to me like this would make sense as an option on the msgpack codec itself.
Rather than having a separate codec (which is also fine but should leave in its own repo than).

Is this considered alive by the author (after 5️⃣ years)?

[moshen]

I don't know about "alive", since I've archived my other logstash projects since I haven't used it in years. I think I made this a separate codec because everything else depends on a newline delimiter, but I may be wrong.