Proposal to make django-oss-storage work with aliyun CDN

[favoyang]

The current implementation assigns OssMediaStorage.location from settings.MEDIA_URL (default value is '/media/'), and use that to construct the OSS URL. This is semi-correct, unless you work with aliyun CDN which use your domain as MEDIA_URL (for example, MEDIA_URL = 'https://files.yourdomain.com/').

Proposal,

• Add settings.OSS_MEDIA_LOCALTION = '/media/'
• Add settings.OSS_STATIC_LOCALTION = '/static/'
• For private acl bucket, use signed_url
• For public / public-read acl buckets, use MEDIA_URL or STATIC_URL when available, otherwise fallback to standard oss url.

The pull request #16 is based on pull request #10 from @jxltom, which added public, public-read acl support.