Update sample software definition for libpng to 1.5.18

The default version set for libpng (1.5.17) does not exist in the source
provided. This commit changes the version to 1.5.18 and the md5 hash to match
the newer version.
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 commit does not include a
deprecation yet. That will be done in a future commit.
Fix @todo comment in Software
Freeze the DEFAULT_CONFIG constant
Remove duplicate test in publisher_spec
Remove unused Artifact methods
Make a digestable mixin
Move checksums into the Package class
Add copyright header to Logger
Merge artifact and package into a single class