[TREZOR] Added Segwit support.

Following changes were necessary outside the TREZOR plugin.
- transaction.py: update_transaction handles segwit transactions.
- keystore.py: added a segwit parameter to bip44_derivation,
  use m/49' instead of m/44' for segwit.

lib/base_wizard.py

@@ -227,7 +227,7 @@ class BaseWizard(object):
             self.derivation_dialog(f)
 
     def derivation_dialog(self, f):
-        default = bip44_derivation(0)
+        default = bip44_derivation(0, self.config.get('segwit'))
         message = '\n'.join([
             _('Enter your wallet derivation here.'),
             _('If you are not sure what this is, leave this field unchanged.')