Support EWS URL hint to avoid long start-up times

[hershi]

See #69
The long auto-discovery time is a known issue with EWS. Hardcoding the URL is an interesting approach, and if it works that's great. It may be a bit less resilient to backend changes though, so it would be a nice-to-have feature to be able to specify an optional URL, and have M2B try connecting with that one, and falling back to auto-discovery if it fails.