GPIO port numbering for rev 1 boards vs rev 2 in Adafruit_CharLCD.py

[grrussel]

I needed to alter the pin number from 27 to 21 for this to work on my rev1 Raspberry PI, it would be nice if there was a revision check added to make this work "by default" regardless of the board revision e.g. as discussed at

http://raspi.tv/2013/rpi-gpio-basics-2-how-to-check-what-pi-board-revision-you-have

diff --git a/Adafruit_CharLCD.py b/Adafruit_CharLCD.py
index 8305243..a8c0dd1 100644
--- a/Adafruit_CharLCD.py
+++ b/Adafruit_CharLCD.py
@@ -53,7 +53,7 @@ class Adafruit_CharLCD:
   LCD_5x8DOTS          = 0x00


-  def __init__(self, pin_rs=25, pin_e=24, pins_db=[23, 17, 27, 22], GPIO = None):
+  def __init__(self, pin_rs=25, pin_e=24, pins_db=[23, 17, 21, 22], GPIO = None):
     # Emulate the old behavior of using RPi.GPIO if we haven't been given
     # an explicit GPIO interface to use