mg811: Initial implementation

This driver was developed on a DFRobot CO2 sensor based on the MG811:
http://www.dfrobot.com/index.php?route=product/product&product_id=1023

It seems to *require* very precise calibration at 400ppm and 1000ppm
to be accurate.  It also gets pretty hot (due to the heater) and
consumes significant current.

Signed-off-by: Jon Trulson <jtrulson@ics.com>
Signed-off-by: sisinty sasmita patra <sisinty.s.patra@intel.com>

examples/python/mg811.py

@@ -0,0 +1,51 @@
+#!/usr/bin/python
+# Author: Jon Trulson <jtrulson@ics.com>
+# Copyright (c) 2015 Intel Corporation.
+#
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject to
+# the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+import time, sys, signal, atexit
+import pyupm_mg811 as sensorObj
+
+# Instantiate an MG811 on analog pin A0, and digital pin D2 with an
+# analog reference voltage of MG811_AREF (5.0)
+
+sensor = sensorObj.MG811(0, 2, 5.0)
+
+## Exit handlers ##
+# This function stops python from printing a stacktrace when you hit control-C
+def SIGINTHandler(signum, frame):
+	raise SystemExit
+
+# This function lets you run code on exit
+def exitHandler():
+	print "Exiting"
+	sys.exit(0)
+
+# Register exit handlers
+atexit.register(exitHandler)
+signal.signal(signal.SIGINT, SIGINTHandler)
+
+# Every tenth of a second, sample the sensor and output it's
+# detected CO2 concentration in parts per million (ppm)
+
+while (1):
+        print "CO2 concentration in PPM: ", sensor.ppm()
+	time.sleep(.1)