Add generated man pages.

Signed-off-by: Loui Chang <louipc.ist@gmail.com>
Make asciidoc 'optional'

Run `make doc` in the build directory after editing the asciidoc to
regenerate the manpages. Changes to asciidoc source and generated man
pages will need to be commited together.

Signed-off-by: Loui Chang <louipc.ist@gmail.com>