Allow set :fallback, nil to disable fallback MIME type

I was having trouble getting `wants.other` to work and determined that the reason was that it was always falling back to HTML. This patch makes it possible to do `set :fallback, nil` to disable the fallback behavior (see [here](https://github.com/synctree/sinatra-echo/blob/master/app.rb) for an example).

In the process, I bumped the version to 0.2.0, moved the gemspec to its expected location at rack-conneg.gemspec, and removed the dependency on Rake from the gemspec.