using STATIC_* and MEDIA_* settings the django way

[twisker]

set OSS storage location based on STATIC_ROOT and MEDIA_ROOT, other than STATIC_URL and MEDIA_URL.

*_URL means for the template to determind where to find the resources online, while *_ROOT means for the server to locate where the physical resources are. thus, the storage location should be determinded by *_ROOT.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ twisker
❌ 王颖

---

王颖 seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[twisker]

@huiguangjun @favoyang @TonyPythoneer please update pypi with these changes if possible. otherwise, this storage backend is merely useful in a product django project. thanks.