Add a method for getting releases in a bundle matching certain criteria.

Review Request #7732 — Created Oct. 25, 2015 and submitted — Latest diff uploaded


This introduces a convenience method on PackageBundle that looks up the
newest release matching a version range across all channels meeting the
specified criteria. The caller can specify the allowed channel type
(pre-release or release), the release types (alpha, beta, etc.), and
limit the result to the version range provided.

This is useful primarily for dependency processing.

As part of this, PackageChannel, and the serialization format, now
supports a channel type: "prerelease" or "release".

Unit tests pass.