Added "cwd" option to the exec when the project is created

When using this module it didn't worked for me until I added this option, for example:

    composer::project { 'satis':
      project_name   => 'composer/satis',
      target_dir => "$home/satis",
      version=> '1.0.0-alpha1',
      stability  => 'dev',
      keep_vcs   => "$home/satis",
      dev=> false,
      working_dir=> $home,
      user   => 'satis',
    }

Failed miserably with permission errors.