Fix for https://github.com/zenchild/Viewpoint/issues/84

The input format for extended_properties via extended_field_uri is possible in a few variations see http://msdn.microsoft.com/en-us/library/exchange/aa564843%28v=exchg.150%29.aspx

The current implementation does not work <at all>.

This commit adds support for extended_property requests via PropertyName, while definitely incomplete it is the step into the direction to be taken.