jackson-datatype-guava: Add PackageVersion to improve Android memory usage

This builds on the support in
https://github.com/FasterXML/jackson-core/pull/49 to improve Android
memory usage for apps that use jackson-datatype-guava.
Refactor shared properties and plugin logic into oss-parent.

Revert changes to .gitignore now that generated sources are under target/ dir.

Now depends on com.fasterxml oss-parent 5-SNAPSHOT.
Add a unit test.
Make PackageVersion.java.in implement Versioned and add comments.
Merge pull request #16 from bgertzfield/master

jackson-datatype-guava: Add PackageVersion to improve Android memory usage