set ETag to overwrite when updating or deleting a registration

The ETag needs to be set to "*" when updating or deleting a
registration, otherwise it returns
"Error 412 Precondition Failed–ETag header missing or invalid"

This is according to the code in Microsoft.WindowsAzure.Messaging.dll,
which is the Notification Hub client for Windows Phone
Merge pull request #7 from nitescua/master

set ETag to overwrite when updating or deleting a registration