Support JavaDoc code tags.

[serac]

Adds support for the following JavaDoc code styles:

1. <pre>{@code}</pre> - Multi-line snippet surrounded by ....
2. <code></code> - Single-line code snippet surrounded by backticks
3. {@code} - Single-line code snippet surrounded by backticks

Fixes #346

[serac]

The build appears to have produced the changes to documentation and license headers, so presumably that's acceptable. Also, I only added support for the JDK9 doclet since that's the only use case I have for this feature.

[jmisur]

FYI the intellij-codestyle.xml was pushed to master and codebase should be now reformatted. If you use that, there won't be any more license header diffs.

[serac]

Would you like me to rebase against master and resubmit to address license header issues? I'll try to respond to requested code changes/test coverage by end of next week.

[fbenz]

Archiving the project