Add CachedStaticSwiftStorage and ManifestStaticSwiftStorage

[leo-naeka]

This pull request adds both CachedStaticSwiftStorage and ManifestStaticSwiftStorage, based on django.contrib.staticfiles equivalents : CachedStaticFilesStorage and ManifestStaticFilesStorage.

There's not much code added, it mainly relies on staticfiles' mixins, but there's 2 notable changes:

• It normalizes the path, because some path may be relative and swift evaluates paths as a whole, nor like a filesystem where myapp/css/../img/background.png is myapp/img/background.png.
• The ManifestStaticSwiftStorage overrides read_manifest to handle ClientException which indicates that the manifest doesn't exist (like IOError on filesystem).

[KostyaEsmukov]

Instead of silently swallowing the exception I would suggest to wrap it in IOError instead, like this (and maybe in the StaticSwiftStorage._open method instead of ManifestStaticSwiftStorage.read_manifest ):

except swiftclient.exceptions.ClientException as e:
    # django.contrib.staticfiles.storage.ManifestFilesMixin#read_manifest
    # When manifest file doesn't exist, the raised error must be an instance of IOError.
    raise IOError(e)

If you want to ignore the missing manifest error, it's possible by setting ManifestStaticSwiftStorage.manifest_strict property to False. So something like this will allow to make the ignoring of the error configurable:

class ManifestStaticSwiftStorage(ManifestFilesMixin, StaticSwiftStorage):
    manifest_strict = setting('SWIFT_MANIFEST_STRICT', True)