Server certificate validation for APNS and GCM

Added server certificate validation to the ApplePushChannel and
GcmPushChannel classes (default off).

The ApplePushChannelSettings and GcmPushChannelSettings classes define a
boolean flag (ValidateServerCertificate) which is used by the
ApplePushChannel and GcmPushChannel classes (respectively) to determine
whether to validate certificate policy errors or ignore policy errors
for remote server certificates