Handle tracing flags on encode/decode
Merge branch 'master' into tracing-support

Conflicts:
	cassandra/decoder.py
First draft of QueryTrace, TraceEvent classes
Fix bad trace body read, convert to UUID
Add __str__ for trace classes, friendlier types
Enable tracing through execute(_async)() kwarg only
Documentation for new tracing classes and attrs
Add method to retrieve query trace
Remove tracing from TODO list in README