debian: use minimal dh rules

`build` target is already `dh build` by the generic target. Same for the
`clean` target which will in turn invoke `dh_auto_clean` and friends.
debian: don't use a native package
debian: use the dedicated Homepage field to point to Serf homepage
debian: let dh_installinit install Upstart service
debian: provide an init.d script

Unfortunately, this init script will just throw away any logs from
serf. There are several ways to work around this, one of them is to
include syslog support in serf. Another one is to redirect logs to a
file (with svlogd for example).
debian: make serf runs under its own user/group
debian: update upstart script to also use _serf/_serf user
debian: use /etc/serf instead of /etc/serf.conf

Moreover, ship the configuration as an example.
debian: remove second upstart.conf in favor of the one in debian

Since only Ubuntu and derivatives are using upstart.conf, let's prefer
the one introduced in debian/ directory.
debian: use "respawn limit" to avoid fast restart of Serf for Upstart
Merge pull request #188 from vincentbernat/fix/debian

ops-misc: Few fixes for Debian packaging