[MSHARED-270] Add Implementation-URL to DefaultImplementationEntries

- Modified source, test, docs
- java.util.jar.Attributes has one unpleasent drawback: It is backed by a HashMap, so insertion order of entries is not retained and Implemention-URL lands in the middle of nowhere.
- Note to m-(javadoc|jar|etc.)-plugin implementors: if your are using 2.6 and plexus archiver, you *must* upgrade plexus archiver to 2.2 otherwise your plugin won't compile due to a missing symbol

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1486160 13f79535-47bb-0310-9956-ffa450edef68