v2.0.0-alpha.3: Immutable manifest references, logging

The primary features in this preview are immutable manifest references and
context aware logging. A new README and a documentation outline are accompanied
by several stability improvements, as well.

v2.0.0-alpha.2: Notifications and stability improvements

This third alpha release includes support for webhook notifications. Several
stability improvements are included that address common errors. A debug server
with pprof and expvar can now be enabled.

The build system has been improved, as well.

v2.0.0-alpha.1: Bugfixes, godep and storagedriver improvements

This second alpha release provides fixes for s3 IAM role authentication in
addition to some stability improvements in storage driver. This is still alpha
quality software and should be run with that in mind.

v2.0.0-alpha.0: Docker 1.5 testing revision

This tags the version of the registry used for integration testing in Docker
1.5. This is a pre-release and should not be used without care.