Issue 45: Added support for Trireme to consult an optional extra ClassShutter.

This will allow Trireme embedders the ability to open up Rhino for passing/using
Java objects in their script environments.
A few tweaks to the RhinoContextFactory.
Move tests to "node10src" so that we can start to split apart different
modules for different Node versions.
Change the dependency tree -- "node10src" depends on "core," and
merge "spi" with core.
Continued work on handle refactoring.
More work on separate handle classes and handle cleanup.
Clean up pin/unpin for sockets and other types of handles.
Initial support for running multiple implementations of Node.js in the
same JVM.
Continued work on new handle implementation for Node 11.
More work on handles.
More attempts at separating this which are not working.
Merge in changes to support multiple Node.js versions in one instance
of Trireme. Node 10 is still the only version supported right now.
Fix pin/unpin problem with streams.
Update README for new package structure.
Update release notes for 0.7.1.
[maven-release-plugin] prepare release trireme-0.7.1
[maven-release-plugin] prepare for next development iteration