Set up code owners to avoid accidental merges

[karalabe]

GitHub recently introduced the concept of code owners, where any PRs modifying owned paths requires a review from the owners. Although I'd like to avoid over-engineering our review processes, it's important for critical packages (e.g. core) to be thoroughly reviewed. The code owners feature would give a heads up to relevant people that a large PR might have made some tiny modifications in critical code that needs special care.