Use newer native "write HTTP" plugin to send metrics to Librato, if configured.

Add a status vhost to apache, so that monit's http monitoring will work.
It doesn't particularly matter to the monit check what this vhost does
as long as it returns 200, but I thought it would be nice to use
apache's builtin status functionality.  Ideas cribbed from [1].  It
might also be possible to use monit's apache-status functionality to
alert on more sophisticated criteria, but this will do for now.

Open question: does collectd support apache-status? Might it also be
interested in this vhost?

Fixes #299.

[1] http://mmonit.com/wiki/Monit/MonitorApacheStatus

The tomcat6 user doesn't have the right privileges to run the
/etc/init.d/tomcat6 script.  Removing these lines allows monit to
restart tomcat if it stops for any reason, and makes the tomcat6 monit
config more consistent with other monit configs elsewhere.

This change set builds collectd from source and configures it in one of
the following ways:

- If Librato credentials are present, collectd will be configured to
send data points to Librato using the collectd-librato plugin.

- If no Librato credentials are present, collectd will be configured to
write RRD files locally (/opt/collectd/var/lib/collectd/rrd by default).

added dovecot-pop3d
allowed in the firewall
monitored with monit
added relevant tests

Remove all configuration for MySQL and configure PostgreSQL as the main
database.

All *_mysql_* options have been changed to *_db_* options.

Postgres requires the database user to have a password in order to
connect via localhost. The db_admin_password option is used to set the
password of the admin user (usually postgres).

Web interface listens on localhost only.
The web interface is needed for polling monit
status from the console via::

    monit summary