added initial interrupt example for activity

Author: siddacious

examples/activity_interrupt.py

@@ -0,0 +1,12 @@
+import board
+i2c = board.I2C()
+
+import adafruit_lsm303_accel
+accel = adafruit_lsm303_accel.LSM303_Accel(i2c)
+
+accel._act_threshold = 20
+accel._act_duration = 1
+accel._int2_activity_enable = True
+
+# toggle pins, defaults to False
+accel._int_pin_active_low = True