Write some documentation for SFTPStorage

http://django-storages.readthedocs.org/en/latest/backends/sftp.html made me sad, so I decided to fix it.

The docs are based on the comment at the top of sftpstorage.py, with some minor edits.  I haven't personally used SFTPStorage yet, so there may be some inaccuracies.  Caveat reader.