Refactor "omnibus release" into a non-S3-specific thing

The current release command is heavily tied to S3. This commit extracts the package-specific bits into a new `Package` class, which removes much of the logic from the S3 releaser. These methods are now available in future releasers.

This commit renames the CLI command from "omnibus release" to  "omnibus publish". As a team, we agreed that "release" was an incorrect verb for the action taking place. NOTE: this PR does not include a deprecation yet. That will be done in a future commit.

/cc @opscode/release-engineers