Add `yo` as peerDependency

By adding `yo` as peer dependency, the user only has to `npm install -g generator-firefox-os` and is good to go.

I also added `npm >= 1.2.10` as engine requirement which shows a non-fatal warning in case the user is using an earlier version of npm, which doesn't support peer dependencies yet.

/ref https://github.com/yeoman/generator/issues/305