Azure backend

[nitely]

This PR supersedes #318. Implements:

• Upgraded azure dependencies (azure-storage is no longer the recommended lib, this depends on azure and azure-storage-blob instead)
• Support for streams to avoid hitting disk (although Django may hit the disk if regular form file is used)
• Proper file name checking
• listdir method to list files by virtual path
• Several settings that were missing in previous PR: timeout, upload_max_conn, overwrite_files, location and default_content_type. See docs for the settings explanation.
• Integration tests using azurite (oficial emulated Azure).
• Docs explaining how to use public and private storage.

[gilgamezh]

🎉

[jedie]

Will it then be v1.7? Please write here when it comes out!

[jschneier]

@jedie version 1.7 is now live.

[jedie]

Thanks for the update!