Improve common.py.

- Modify path_join(), so that with inputs 'foo/', 'bar', it will return 'foo/bar',
  not 'foo//bar'.

- Add path_join_SEP(), which returns a path ends with '/', we can use this
  method to get directory path (maybe specific for s3 only)

- Add check_object_basename() to check the object basename correctness.
Adds new features to django_cloud_browser AWS S3.

- New features:
    - Upload a file.
    - Create a new directory.
    - Delete a file or a directory.
    - Rename a file or a directory.
    - Move a file to another directory.

- Add generic methods to base.py.

- Realize generic methods for AWS under cloud/boto_base.py, and upload form
  under cloud/aws.py

- Add upload, createdir, delete, rename, move, view (some code might be still
  specific for S3, need to improve it)

- Add rename, move template files.