Update various Maven details.

[bturner]

• Added and configured nexus-staging-maven-plugin based on current publishing instructions
• Added mvnvm.properties, which can be used with mvnvm to indicate what Maven version to run
• Added explicit versions for relevant Maven plugins and updated all of the existing versions to current
• Moved configuration for the Javadoc and source plugins to the release profile
• Removed the oss-parent POM
  • Current publishing instructions for Central indicate the oss-parent POM is deprecated/unsupported and should not be used
• Ran mvn tidy:pom to apply best-practice ordering
• Renamed release-sign-artifacts profle to release-profile, matching the name of the default release profile
• Replaced org.hamcrest:hamcrest-junit with hamcrest, and updated to 2.2
• Updated JNA to 5.11, but modified the OSGi instructions to set the lower bound to 5.5 (matching the 2.0.0 release)

[bturner]

I've done a trial 2.0.3-rc1 with these changes, and then dropped the staging repository and tag rather than publishing.

@brettwooldridge I'm going to merge this tomorrow so I can do a 2.0.3 release. If you have time before then to glance at it, great. If not, totally understandable. If there's anything you see here you don't like whenever you do have time to take a look, just add comments and I'll circle back to it in a follow-up pull request. (I've already taken way longer to get to where I think I can release 2.0.3 than I intended, so now I'm hoping to turn it around quickly.)

[bturner]

I've rebased my pull request to sign all of my commits. I've also added a property to allow specifying whether to auto-release staged artifacts or not (defaulting to true, though I'll probably override it to false when I release 2.0.3 just to be safe)