Codecs - Build out framework

SO EXCITING.

I'm marking this fixed as I have successfully been able to write my own codec (multiline).

WOot.

1.2 is shining up nicely!

Proposed solution:

A codec will be another logstash plugin, but it will not have its own config block. Instead inputs and outputs will have a 'codec' paramater similar to how the current 'format' parameter is today. Many codecs will exist and be available to users, this will allow all intputs/outputs to benifity from features like batching, compression, encryption, etc...

To implement a codec, you must subclass LogStash::Codecs::Base and override the encode and decode methods. Decode is called when an input has data it wishes to turn into an event. It can pass additional field to the codec that it wishes to include in the event. Encode is called on the output when an event is emitted. The output must define an on_event block that the codec can call when it has data to push through the output. Both inputs and outputs must call enable_codecs to initialize the codec before use.

A proof of concept implementation can be found in this branch:
https://github.com/logstash/logstash/tree/codec-hacking
Compared to master:
https://github.com/logstash/logstash/compare/codec-hacking