fix Helpers visibility to be "internal"

looks like they're "public" by mistake
removed SDKBuild\MyToolkit.dll

it is not needed anymore
sealed class NotificationService

this fixes Windows ACK error: "Type PushSDK.NotificationService is not
sealed and does not have the ComposableAttribute"
fresh SDKBuild
Merge pull request #2 from ringoz/master

fixed minor code issues and rebuilt SDKBuild binaries