Fix upstart scripts to only start when dependent services are up

Should ensure that after running `vagrant reload`, the `celeryd`, `tiler`, and `ecoservice` jobs will be running.

Fixes #21 
These are fairly standard Upstart events (http://manpages.ubuntu.com/manpages/utopic/en/man7/upstart-events.7.html).  While it's confusing to someone who hasn't written an Upstart script, I think it's pretty standard stuff that doesn't deserve a comment in the file itself.

Perhaps we should create a `CONTRIBUTORS` document to house links to thses sorts of things?