feat(dbus): make API synchronous and remove assumptions

Previously, this library assumed all jobs would be started/stopped/etc
with "replace" as the mode string.  There is no reason to make this
assumption.
feat(dbus): add transient units support
chore(dbus): reorganization/clean up
docs(dbus): minor StartUnit clarification
Merge pull request #8 from sakana/master

feat(dbus): API change and transient unit support