Add support for EWS to support up-to-date MS Exchange

Currently there's only older versions of Exchange supported.
But I guess quite some people, as myself, need MS Exchange support for their works accounts.

There's been a discussion about this in the K9 repo, but it seems it's not being worked on. I guess this could be contributed upstream...
https://github.com/k9mail/k-9/issues/606

One suggested way is to use MS provided API: https://github.com/OfficeDev/ews-java-api