Release v0.2.0

Major breaking changes:
- thriftbp.ClientPool now implements thrift.TClient, and no longer
  provides GetClient and ReleaseClient APIs.
- Mocks and test helpers from thriftbp package are moved into new
  thriftbp/thrifttest package.
- When doing logging with a context object, you should use log.C(ctx)
  instead of top level log package functions.

Other breaking changes:
- metricsbp.Statsd.Statsd is no longer exported.
- thrifttest.CopyTStruct (previously thriftbp.CopyTStruct) now has its
  args order flipped.

Release v0.1.0

First tagged release!

The API is starting to stabilize. We still may (and likely will) do
breaking changes before hitting the first major version release, but
those breaking changes will be less frequent and less dramatic.