Add `yo` as peerDependency

By adding `yo` as peer dependency, the user only has to `npm install -g generator-yo-wordpress` and is good to go. We're currently in process of adding this to all official generators, so we can adjust the documentation accordingly.

https://github.com/yeoman/generator/issues/305

*Edit*

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.