Adds improved `ember build --watch` and tests.

This is `ember build --watch` take 3.

* options.watch => --watch flag, which is false by default.
* Build command chooses the task based on options.watch.
* Watch Task is in build-watch.js.
* Unit test in cli-test.js verifies that watch option is passed correctly.
* Acceptance test in smoke-test-slow.js, verifies that changes to file
  are written to rebuilt file by --watch.