Storage classes can have prefixes

Adds a `prefix` kwarg to `CloudFilesStorage`, which specifies a string
to be prefixed to the filenames saved with the instance. Also adds
`PREFIX` and `STATIC_PREFIX` settings, which are the default prefix
values for `CloudFilesStorage` and `CloudFilesStaticStorage` instances,
respectively.

This fixes `CloudFilesStaticStorage.save` and
`CloudFilesStaticStorage.url`, which use unprefixed names, making them
incompatible with the syncstatic command (which always used `STATIC_URL`
as a prefix).
Only delete files with specified prefix

This essentially gives you virtual buckets within your container. For
example, you could upload your media and static files to a single
container--prefixed with 'media/' and 'static/', respectively--and run
the syncstatic command without deleting any media files.
Add prefix settings to documentation
Add modified_time method to storage class
Merge branch 'storage-prefixes'
Merge branch 'modified-time'