Ensure we have the right 's' prefix for the SOAP elements.

Previously, we let ElementTree pick its own prefix, and it took 'ns0'.
Log request errors.

This is much needed when you get 500's, which usually come with an
explanation on why the request was not accepted.
Fix getting a calendar event list with Exchange 14.2.247.0

The SOAP query was returing the following error:

    The request failed schema validation: The 'EndDate' attribute is
    invalid - The value '2014-10-28' is invalid according to its
    datatype 'http://www.w3.org/2001/XMLSchema:dateTime' - The string
    '2014-10-28' is not a valid XsdDateTime value.

The fix is simply to use the DATETIME format instead of the DATE format,
as advised in the MSDN page of the CalendarView element.

http://msdn.microsoft.com/en-us/library/office/aa564515%28v=exchg.140%29.aspx
Merge pull request #24 from nud/fix-list-events

Fix getting a calendar event list with Exchange 14.2.247.0 …