def initialize(gem, options = {})
    @gem               = gem
    @version           = options[:version] || Gem::Requirement.default
    @gem_home          = File.expand_path(options[:install_dir] || Gem.dir)
    @force_executables = options[:executables]
    @force_all         = options[:all]
    @force_ignore      = options[:ignore]
    @bin_dir           = options[:bin_dir]
    @format_executable = options[:format_executable]

    # only add user directory if install_dir is not set
    @user_install = false
    @user_install = options[:user_install] unless options[:install_dir]

    if @user_install then
      Gem.use_paths Gem.user_dir, @gem_home
    else
      Gem.use_paths @gem_home
    end
  end
#
Constructs an uninstaller that will uninstall +gem+