Fix TypeError for PY3 (google#97)

Traceback (most recent call last):
  ...
  File "python-adb/adb/sign_pythonrsa.py", line 30, in update
    self._buf += msg
TypeError: Can't convert 'bytes' object to str implicitly

Author: fgervais

adb/sign_pythonrsa.py

@@ -24,7 +24,7 @@
 # hashing algo for this.
 class _Accum(object):
     def __init__(self):
-        self._buf = ''
+        self._buf = b''
 
     def update(self, msg):
         self._buf += msg