Swapped to using Sinatra::Base

It's good practice to require sinatra/base rather than sinatra in production apps because you have more control from the Rack side of things (which is the ruby HTTP middleware).

For example, it's now possible in your config.ru file to mount this site at `/greg` and then mount a different rack application at `/other` and have all the paths translated for you. You could even mount your blog at two different paths (useful for swapping between an 'old' and a 'new' blog in the future)