Added progressive enhancement to adapters

[tbjers]

Will warn when an adapter is instantiated if its driver is not installed.

[mde]

This looks like a solid approach, but with a local install (e.g., if they npm install) inside their app, the require will have to be a 'local require' -- see the requireLocal method here: https://github.com/mde/utilities/blob/master/lib/file.js. We actually used to do this (that's where the requireLocal method originally came from), falling back to looking for an adapter installed directly in Model. But I think we want to assume there's always a local install (i.e., inside the app where somebody's using Model).