Add `ember build --watch`.

* 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.
* Empty catch comment.
* Updates changelog.
Merge branch 'build-watch'