Do not use method missing on tap-gen manifest

[thinkerbot]

By using method missing you open up subtle bugs:

    m.template_files do |source, target|
      m.file(target) {|io| io << File.read(source) }
    end

Does not work (registers template_files as an action) whereas this is correct:

    template_files do |source, target|
      m.file(target) {|io| io << File.read(source) }
    end

It would be preferable for the first to throw an error or something.