Office365 API mailbox support

[Pietro395]

We have implemented Office365 API support to read mailboxes. On Office365 IMAP is disabled by default and it is necessary to use Oauth, we used the O365 library to implement this mailbox type.

I have updated the documentation with details on how to set up credentials for Office365. You need to register an application and get client_id, client_secret and tenant_id, the steps are documented here: https://github.com/O365/python-o365#authentication

It's our first pull request in a public project, let us know critical issues about our implementation. We hope that the pull request is appreciated and can be implemented in the main brach.