Updated Bernhard's Protobuf (pb.py) to the current Riemann Upstream!

This change includes a few things:

First, the 'protoc' protobuf generator now outputs a pb.py file with slightly different variable naming, resulting in more changed lines than would otherwise be the case.

Second, the updated protobuf file includes more fields - notably a repeated field called 'attributes'.

Third, I modified the Event class to support setting each key in a dictionary as an 'attribute' object on the pb.event.