Forget to importing "re" module
Clean up and add some comments
New way to implment yum S3 plugin via inheriting the '_getFile' API from yumRepo.YumRepository.
Benifits:
  1. Plugin becomes a minimal set and easy for further development and maintenance
  2. No need to handle the raw HTTP(S) request/response itself but managed by Yum
  3. Hand over most dirty tasks tothe more robust urlgrabber of Yum
  4. Plugin only cares about AWS S3 API and its configuration
  5. Good news: most params in yum.conf can be supported since now