Support for an 'end session' protocol message.

1) On the push side, this message is a flag in PushMessageContent.
   Any secure message with that flag will terminate the current
   sessin.

2) On the SMS side, there is an "end session" wire type and
   the convention that a message with this wire type must be
   secure and contain the string "TERMINATE."

library/src/org/whispersystems/textsecure/crypto/AttachmentCipher.java

@@ -31,6 +31,7 @@ import javax.crypto.spec.SecretKeySpec;
 import java.security.InvalidAlgorithmParameterException;
 import java.security.InvalidKeyException;
 import java.security.NoSuchAlgorithmException;
+import java.text.ParseException;
 import java.util.Arrays;
 
 /**
@@ -118,6 +119,8 @@ public class AttachmentCipher {
       throw new AssertionError(e);
     } catch (BadPaddingException e) {
       throw new InvalidMessageException(e);
+    } catch (ParseException e) {
+      throw new InvalidMessageException(e);
     }
   }