Handle large plaintext input properly.

* onetime
  (main): When encrypting, read in 8k chunks, instead of reading the
    whole file into memory.  If someone has enough pad to encrypt
    their DVD collection, we should still perform acceptably.

onetime

@@ -929,7 +929,7 @@ def main():
     output.write(onetime_header)
     encoder = PadEncoder(pad, config)
     while 1:
-      string = incoming.read()
+      string = incoming.read(8192)
       if len(string) > 0:
         result = encoder.encode(string)
         if result: