Moved update policy interpretation from UpdateCheckManager into dedicated UpdatePolicyAnalyzer component to ease maintenance and customization

* Was using platform encoding to write out the checksum files. The checksums should always be
  a subset of US-ASCII, so forcing encoding to US-ASCII.

* Class is final anyway, and the reference to TransferEventCatapult.this is never used.

Fixed EnhancedLocalRepositoryManager to register timestamped snapshot artifacts cached from remote repositories using their remote filename

Extracted DefaultRemoteRepositoryManager.getRepositoryConnector out into dedicated component RepositoryConnectorProvider

Merged DefaultRepositorySystemSession.set*Props() and set*Properties(), overload is not really necessary and causes more confusion than good

Refactored error caching to consequently query new ResolutionErrorPolicy extension point which allows to apply different policies for artifacts vs. metadata when needed

Downgraded maven-jar-plugin to version 2.3.2 to avoid https://github.com/sonatype/m2eclipse-extras/issues/10

Moved Plexus support components of wagon connector into internal package, nothing but an IoC container should ever access those classes

Deleted superfluous StubArtifact and StubMetadata now that DefaultArtifact and DefaultMetadata are part of API