HTML Doxygen Generation

[calgray]

As mentioned in #80:

• INPUT is empty, so doxygen tries to scan the full repository looking for source, which takes a long time (because of all the external code that is checked out). I'd suggest setting this to src for faster turnaround.
• Several of the WARN_* options are set to NO, so it would seem there's nothing wrong. I tried turning on WARN_IF_UNDOCUMENTED, WARN_IF_DOC_ERROR and WARN_NO_PARAMDOC and that resulted in 488 warnings.
• The doxygen cmake target seems incorrect: it runs under the src directory in the binary cmake directory, where there's very little to document (only the generated git_revision.cc file).
• The Doxyfile has HTML output turned off, so to test this locally I had to at least turn it on.