Convert manual into AsciiDoc.

AsciiDoc can be more easily converted into other formats, such as HTML, than
troff can, and presentation can be more easily customized as well.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Update Makefile to build AsciiDoc documentation.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>