Import WalletFactory instead of Wallet

WalletFactory and WalletBitkey classes
--usb parameter enables experimental support for bitkey protocol

lib/wallet_factory.py

@@ -0,0 +1,11 @@
+class WalletFactory(object):
+    def __new__(cls, config):
+        if config.get('bitkey', False):
+            # if user requested support for Bitkey device,
+            # import Bitkey driver
+            from wallet_bitkey import WalletBitkey
+            return WalletBitkey(config)
+        
+        # Load standard wallet
+        from wallet import Wallet
+        return Wallet(config)