Add encode capability

[ryananguiano]

Requested by @malonej7 and others.

We need to develop the encode capabilities using Confluent Avro Schema Registry. Luckily schema registration is already taken care of by https://github.com/wvanbergen/schema_registry so we just need to figure out the settings interface.

The options that need to be added:

• subject_name
• schema_uri
• schema_string
• check_compatibility
• override_compatibility

If anyone can think of anything else they would need, feel free to add them here. I might have time to take a crack at this soon unless someone else wants to give it a try. I will be happy to help walk anyone through it.

[ryananguiano]

https://github.com/logstash-plugins/logstash-codec-avro/issues/14#issuecomment-217989763

[cgiraldo]

I have implemented a basic encoder -> #4

[ryananguiano]

I updated the encode method, but still needs to be tested -> #5

[ryananguiano]

Merged onto master