Fixing date-formatting and -parsing problems in SnasphotUtils, and adding methods to help users do the right thing WRT generating snapshot dates (using UTC)...along with formatting/parsing lastUpdated-style date strings for metadata files. Fixing parsing in ArtifactPathInfo when it's a remote snapshot (which introduces two new capture groups for the regex). Adding convenience method getSnapshotPart() to SingleVersion. Adding tests for SnapshotUtils.