Keep --output-path up to date from build and serve commands.

* Adds ability to specify output-path for `ember server` command.
* Removes usage of `broccoli-export-tree` in favor of maintaining a
  single set of logic in the `Builder` model. (previously we had logic
  in both the `Build` task and `loadBrocfile` utility function to keep
  the output directory in sync)
Merge pull request #1034 from rjackson/live-output-to-dist

Keep --output-path up to date from build and serve commands.