minification with uglifier

I'd like to add this as an automatic package to jsdelivr. I've modified the Rakefile to produce a minified version of the file in question, as a minified file must live in the github repo for jsdelivr to accept an automatic package.
This is probably better written as a `file` task:

```ruby
file "lazy.min.js" => [ "lazy.js" ] do |task|
  File.open(task.name, 'w') { |f|
    content = task.prerequisites.map { |prereq|
      if File.exist?(prereq)
        File.read(prereq)
      elsif Rake::Task[prereq].nil?
        raise "Prerequisite #{prereq} does not exist."
      end
    }.compact.join("\n")
    file.write Uglifier.new.compile(content)
  }
end

desc "Concat and uglify JavaScript"
task :uglify => [ "lazy.min.js" ]
```