Ticket #14: typo on doc/milkcheck.asciidoc

This patch fixes #14
ticket #16: Describe returned code in milkcheck man page

This patch fixes #16
doc: move authors directly in the AUTHOR section

asciidoc doesn't support correctly multiple authors on autogenerated AUTHOR
section. Before this patch 2 AUTHOR sections where generated.
Makefile: clean generated manpage and python build directory