[WIP] Display message to user if an ember-cli update is available

This is a first pass at checking for ember-cli updates and displaying a message to the user if so (fixes https://github.com/stefanpenner/ember-cli/issues/335).

## TODOs:
- [ ] some kind of persistence, so it doesn't have to check on every call
- [ ] possible option to permanently dismiss this message
- [ ] possible 'ember update' command?
- [ ] tests