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
Merge pull request #198 from robbcolonthree/master

Server certificate validation for APNS and GCM