fix a compilation error with -Werror=format-security

Debian's dpkg-buildpackage seems to use -Werror=format-security.

This patch fixes this compilation and lets us build i3blocks .deb
packages.

[vd: add the CFLAGS in the Makefile]

Closes #41
Makefile: allow customization of LIBEXECDIR

By default, scripts are installed in /usr/libexec, but on Debian based
system, this directory is not used. /usr/lib is used instead.

I have added LIBEXECDIR to Makefile (which defaults to /usr/libexec) to allow
customization.

Closes #42
Makefile: fix manpage uninstall

[vd: split from Jason's PR43]
Makefile: install the content of the contrib folder

Added DOCDIR (defaulting to $PREFIX/usr/share/doc/$PROGRAM), and
installed the contents of contrib/ to $DOCDIR/contrib. Files are set to
755 except config, which is set to 644.

[vd: move up DOCDIR declaration]

Fixes #28
Closes #43
travis: install ruby-ronn