Issue #6: Null safety.
Issue #6: First pass at thread safety documentation.

This is by no means complete or even necessarily correct, it just involves
a best-guess based around whether it looks like it was intended to be thread
safe, not thread safe, or immutable.
Issue #6: Header cleanup.
Merge branch 'master' of github.com:dclements/revori into jsr305
Merge pull request #11 from dclements/jsr305

Issue #6: JSR 305 integration.