Add documentation to pljava-so #525

There is currently no documentation in the pljava-so module.

Doxygen is a widely used documentation tool.

The pull request associated with this issue includes a 'proof of concept' implementation of doxygen documentation. I made no changes to the existing files - but did turn on a ton of options that should probably be disabled in the released version.

Requirements

• doxygen package
• graphviz package

Usage

$ cd pljava-so
$ mkdir target
$ cd src/main/resources
$ doxygen

Notes

I've noticed a few glitches in the inlined source code. E.g., a 4-line macro that somehow had several hundred addtional lines added to the snippet. This may be a configuration issue or it might be a bug in doxygen. I just wanted to call it out since it's present, albeit rare.