v2.0.0-rc.4: Optional resumable digest, logging adjustments, documentation

A build flag is now avialable to disable resumable digest support. This allows
installations that cannot use the forked crypto library to revert to using Go's
stdlib. Several fixes have been made to contextual logging output, including
ensuring accurate http response status, authorized user name and correct output
of the version.

The documentation continues to be shaped and massaged.

v2.0.0-rc.3: Driver tracing, documentation, and bugfixes

The main addition in this release candidate is around logging storage driver
calls with performance data and ensuring that log messages have nanosecond
granularity timestamps. Other bugfixes affect upload resource cleanup.

With this candidate, the documentation has undergone a large amount of
refactoring, including additions with various production setups.

v2.0.0-rc.2: Performance enhancements, bugfixes

This is an updated release candidate that includes a performance enhancement
for fetching manifest signatures. The backend path matching validity has been
relaxed to defer case-sensitive support to the storage backend.

v2.0.0-rc.1: Documentation, layer caching and bugfixes

This is an updated release candidate that includes layer info caching and some
bugfixes for cloudfront support. Documentation has also been heavily updated
but is still not complete.

v2.0.0-rc.0: Initial release candidate

This is the first release candidate for Registry 2.0. Since the alpha release,
we've mostly focused on hardening and refactoring to bring the software up to
production quality.

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.