Add possibility to set default sizes.

This change adds two new parameters to the constructor of the
Transport class. Letting the user set the default windows size and
default max packet size for the instance.

paramiko/transport.py

@@ -135,7 +135,10 @@ class Transport (threading.Thread):
 
     _modulus_pack = None
 
-    def __init__(self, sock):
+    def __init__(self,
+                 sock,
+                 default_window_size=64 * 2 ** 15,
+                 default_max_packet_size=2 ** 15):
         """
         Create a new SSH session over an existing socket, or socket-like
         object.  This only creates the `.Transport` object; it doesn't begin the
@@ -163,6 +166,12 @@ def __init__(self, sock):
 
         :param socket sock:
             a socket or socket-like object to create the session over.
+        :param int default_window_size:
+            sets the default window size on the transport. (defaults to
+            2097152)
+        :param int default_max_packet_size:
+            sets the default max packet size on the transport. (defaults to
+            32768)
         """
         if isinstance(sock, string_types):
             # convert "host:port" into (host, port)
@@ -231,8 +240,8 @@ def __init__(self, sock):
         self.channel_events = {}       # (id -> Event)
         self.channels_seen = {}        # (id -> True)
         self._channel_counter = 1
-        self.default_max_packet_size = 2 ** 15
-        self.default_window_size = 64 * self.default_max_packet_size
+        self.default_max_packet_size = default_max_packet_size
+        self.default_window_size = default_window_size
         self._forward_agent_handler = None
         self._x11_handler = None
         self._tcp_handler = None