Robust clone

Try to make `install-*` plans a bit more robust:
- give a helpful message if `xtuple` hasn't been checked out yet
- be more forgiving of the state of other repositories
- be smarter about `config.js` over multiple runs

This will not work nor pass CI until #494 is merged and `npm publish`ed.
[ edit - the #494 changes originated on this branch and were pulled out to allow separate merge/test/publication ]
This uses [node's assert](http://nodejs.org/api/assert.html) library, not chai.  `isUndefined()` and `isNull()` don't exist.