Proto commits in Yelp/py_zipkin

These 3 commits are when the Protocol Buffers files have changed:

Commit:933b069
Author:Daniele Rolando

fix flake8 issues

The documentation is generated from this commit.

Commit:c4369bc
Author:Daniele Rolando
Committer:Daniele Rolando

Support PRODUCER and CONSUMER spans

Commit:4a957a4
Author:Daniele Rolando
Committer:Daniele Rolando

Add protobuf encoding This PR adds support for the protobuf v3 encoding. I tested this by manually generating a few traces, writing them to a file and sending them to zipkin with curl. I'm checking in the zipkin.proto file. It's not really necessary but it seemed to make sense to keep it next to the generated code. I also changed `encoding.create_endpoint` to accept an `use_defaults` param which disables guessing default values. That makes tests easier since the guessed ips would change from host to host.