[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/keystore.py

@@ -684,11 +684,10 @@ is_private_key = lambda x: is_xprv(x) or is_private_key_list(x)
 is_bip32_key = lambda x: is_xprv(x) or is_xpub(x)
 
 
-def bip44_derivation(account_id):
-    if bitcoin.TESTNET:
-        return "m/44'/1'/%d'"% int(account_id)
-    else:
-        return "m/44'/0'/%d'"% int(account_id)
+def bip44_derivation(account_id, segwit=False):
+    bip  = 49 if segwit else 44
+    coin = 1 if bitcoin.TESTNET else 0
+    return "m/%d'/%d'/%d'" % (bip, coin, int(account_id))
 
 def from_seed(seed, passphrase):
     t = seed_type(seed)