Public API for pub

[stevenroose]

I think it should be possible to access pub using an API.

This will enable a lot of useful developer tools like notifications when a dependence changes to a newer version or even CI to automatically test your app/lib against new versions.

I've been in the situation more than once that I depended on multiple packages that had inconsistent dependencies because one old package did not update it's dependencies and a new package has been created after expiration of the max constraint of the old package. Giving library developers tools to manage the dependencies of their libraries can solve this problem.

Would also solve #253.