Fix initialization of usb2can Bus interface object (#501)

* Fully initialize usb2can Bus interface object.

The usb2can interface backend currently does not work because the
_filters member is not initialized in the constructor of
can.interfaces.usb2can.Usb2canBus.  Call the parent class constructor
to pull in the missing object attributes.

* Add a note about the 32-bit only CANAL DLL.

The usb2can interface currently only supports 32-bit Python on
Windows, because there is no 64-bit CANAL library available yet.
Mention this in the documentation section.

Author: acolomb

can/interfaces/usb2can/usb2canInterface.py

@@ -119,6 +119,8 @@ def __init__(self, channel, *args, **kwargs):
 
         self.handle = self.can.open(connector.encode('utf-8'), enable_flags)
 
+        super(Usb2canBus, self).__init__(channel=channel, *args, **kwargs)
+
     def send(self, msg, timeout=None):
         tx = message_convert_tx(msg)
         if timeout:

doc/interfaces/usb2can.rst

@@ -26,6 +26,7 @@ WINDOWS INSTALL
     1. To install on Windows download the USB2CAN Windows driver.  It is compatible with XP, Vista, Win7, Win8/8.1. (Written against driver version v1.0.2.1)
     2. Install the appropriate version of `pywin32 <https://sourceforge.net/projects/pywin32/>`_ (win32com)
     3. Download the USB2CAN CANAL DLL from the USB2CAN website.  Place this in either the same directory you are running usb2can.py from or your DLL folder in your python install.
+       Note that only a 32-bit version is currently available, so this only works in a 32-bit Python environment.
         (Written against CANAL DLL version v1.0.6)
 
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