DateFormat enhancement and utc

Hi,

I've used Nlog a lot and read your issue
https://github.com/harouny/NLog.Extensions.AzureTableStorage/issues/8

The inline comments sum up where I'm coming from with, repeated below
//logging as UTC time so would be consistant across geo locations
//format is not quite UTC as didn't want to lose millisecond precision but it is culturally neutral
//it should also allow for predictable parsing when reading this back out of azure storage

Otherwise like this a lot, tidy work.

Regards,
Colum