wallet.synchronizer should exist at construction.

The `self.synchronizer` attribute is accessed before it ever exists when using
`electrum importprivkey` on the command line. This change sets it to None,
which is a valid state, in the wallet's constructor.

lib/wallet.py

@@ -186,6 +186,8 @@ class Wallet:
                 print msg
                 sys.exit(1)
 
+        # This attribute is set when wallet.start_threads is called.
+        self.synchronizer = None
 
         self.load_accounts()