Add CLOUD_BROWSER_CACHE_SECONDS to settings.
Add two generic exception: StorageResponseException and ClientException.

to catch server side error response (http status code), and client side
exception, like non-standard container name.
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 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.

cloud/base.py
    - Add generic methods to base.py.

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

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

Templates:
    - Add rename, move template files.
    - Improve template file _browser.html.
        - Add upload, mkdir, delete, rename, move form.
        - Replace encoding column with modified by.
        - Insert 3 columns (Delete, Rename, Move to).

Icons:
    - Added file operations icon.

Admin site:
    - Add rename, move block div to admin/base.html.
    - Add rename and move admin template files, which are with admin/base.html.
    - Add the file operations urls to admin site.