Show revamp

Complete rewrite of 'meteor show', with tests. See commit messages for more details.

This is part of the work on the Meteor tool to get longform package descriptions from README.md files into 'meteor show'. It is safe-contained enough, that it makes sense to submit it for review.

(I won't merge to devel until all the README work is complete -- note that this is being pulled to a staging branch).
meteor admin change-homepage <package name> <url>

So, it is kind of buried in the poorly-documented admin methods. We generally thought of it as somewhat of an advanced feature, not something most people care about.