Doxygen comments in C++ source

Hey all,

In my last few PRs that modified C++ source, I added Doxygen comments for
the functions I added/modified. Since the codebase is somewhat lacking in
documentation, I was wondering if we could establish some organizational
rules about this. Specifically:

- Any new C++ code must have Doxygen comments
- Any changes to existing C++ code must add Doxygen comments if they're not
present
- This applies to all documentable elements - functions, data (such as
class members and globals), enums, classes, and files when appropriate
- In header (hpp) files, document the element according to its usage
(preconditions, parameters, return value)
- In implementation (cpp) files, document implementation details if
necessary
- If the element only exists in an implementation file, document both
- If the element is trivial and can be determined from name alone, it
doesn't necessarily need to be documented.

Ideally, over time, this will lead to the most commonly touched parts of
the codebase becoming well-documented, making them easier to understand and
modify in the future.

Use of these comments was already present (scarcely) in some places in the
codebase before I started adding it, so this is not entirely out of the
blue. I've worked out an addition to our Travis script that automatically
creates the docs and posts them to github-pages:

https://github.com/brianlheim/supercollider/pull/3

You can see the result of that here:

http://www.brianlheim.com/supercollider/index.html

The Doxygen/Javadoc format is widely used and can be parsed by other
documentation generation tools such as standardese (
https://github.com/foonathan/standardese). Doxygen XML output itself can
also be used by other generation tools.

-Brian