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).

Closes #1031.
leave a quick note, why we don't just `rimraf` the whole dir. (e.g. symlinks)