Add Conn.GetUnitOptions and/or Conn.Cat

Sometimes it is helpful to look at an enabled unit file. I'd rather not go look up the file in /etc/systemd, etc., which is kind of the point of using the Go bindings in the first place. :) At the commandline I can just run "systemctl cat ...". Having an equivalent method on dbus.Conn would really helpful. I expect it would dovetail onto work for issue #77.

As a temporary workaround I am reading the dbus properties I need (after hunting down the mapping from the conf directives) and manually converting them into unit.UnitOption values. From there I can use them or call unit.Serialize to get the unit file. However that's a lot of work, full of risk of getting something wrong (or out of date), for something that I would expect is not an out-of-the-ordinary need.

Start by reading the dbus.Conn API and the work described in issue #77. Compare the requested behavior with the command-line systemctl cat operation and the existing unit.UnitOption and unit.Serialize usage. Done means Conn exposes an equivalent way to retrieve enabled unit contents or options without manually mapping D-Bus properties.