Make it more friendly to install on non-Debian distributions

* make DOCBOOK_XML=... supports providing a custom docbook.xsl
  instead of the default (/usr/share/xml/docbook/stylesheet/nwalsh/manpages/docbook.xsl)
* provide install target via Makefile, supporting $DESTDIR
* minimize debian/rules

This should allow installation of grml-debootstrap on e.g. Fedora + Gentoo.
Thanks: Sebastian Pipping <sebastian@pipping.org> for feature request
Fixes #57 @ github