This commit includes a comprehensive refactoring of the
controller:

- ResourceEventHandlers dispatch work to a workqueue for
  a worker that in turn dispatches work to a separate
  queue for each namespace. For each namespace, there
  is a worker that syncs the various resources.

- The controller uses a SharedInformer, with Informers
  and Listers for each resource type. The Listers are
  handed off to the namespace-specific workers.

- Separate source files contain code for syncing
  Endpoints, Ingress, Service and Secret.

- Added the controller CLI options:
  -namespace: for single-namespace deployments
  -templatedir: to locate the templates for VCL

- The director for templates is given either by the
  templatedir CLI option or the TEMPLATE_DIR env
  variable. This makes it possible to run the
  controller from the command-line, and run tests
  from any working directory.

- The Ingress annotation
  ingress.varnish-cache.org/varnish-svc identifies
  the Service name of the Varnish Service intended
  to implement it. If the annotation is absent,
  then the controller looks for exactly one Service
  in the namespace with the label "app:varnish-ingress".
  If none is found, the Ingress is rejected.
  (This is not yet documented.)

- Docs and examples updated

- Example for single-namespace deployment added.