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

Configured snapshot/release distribution via oss.sonatype.org (cf. https://issues.sonatype.org/browse/OSSRH-2684)

Tweaked configuration for maven-bundle-plugin to workaround bogus warning about superfluous export package instructions (https://issues.apache.org/jira/browse/FELIX-3300)

Changed approach to use packaging=jar and only have the maven-bundle-plugin produce the MANIFEST.MF. For unknown reasons, having maven-bundle-plugin build the JARs when packaging=bundle will exclude target/classes/META-INF/* from the JAR, thereby killing Plexus/Sisu support.

Moved DefaultArtifact, DefaultArtifactType, DefaultMetadata and DefaultDependencyNode into API to improve usability