Update README.md, and rename getProperty(key) -> value(key).

While updating the README.md to reflect that this revision is now TinkerPop3 compatible,
I noticed that the examples in the README used some APIs that changed with TP3, but had
not yet been fully converted in the source, i.e. our wrapper still used TP2 names. Those
have been fixed here, though there are definitely other such cases not yet changed.