Improve reloader coverage

Currently development reloading only supports the 'default' Sinatra paths:
"files defining routes, filters, error handlers and inline templates".

This commit adds reloading to all of `lib`, thus including serializers,
mediators and the other common Pliny stuff, plus anything custom to the app.
Merge pull request #137 from neilmiddleton/better_reloading

Improve reloader coverage