Use `Builder` and `Watcher` models, not old utility functions.
Merge pull request #864 from rjackson/remove-unused-utilities

Use `Builder` and `Watcher` models, not old utility functions.
Allow concurrent `ember test --server` and `ember server`.

Prior to this, both `ember server` and `ember test --server` attempted
to write to (and clear) `tmp/output`. This allows the test task to use a
custom randomly named temp dir instead (which prevents the collision).