Merge pull request #114 from mdissel/master

Fix for #104
Merge pull request #115 from mdissel/master

Update readme for #104
Merge pull request #117 from mdissel/master

Group.Id is not required, make it nullable
Convert string properties to (UTC) DateTime fields

Congifure servicestack to assume every datetime field is UTC (during the
communication with MailChimp)