Merge pull request #18 from caternuson/float_doc

Ensure a float return for lux.

Author: ladyada

adafruit_tsl2561.py

@@ -107,7 +107,7 @@ def enabled(self, enable):
 
     @property
     def lux(self):
-        """The computed lux value."""
+        """The computed lux value or None when value is not computable."""
         return self._compute_lux()
 
     @property
@@ -177,7 +177,7 @@ def _compute_lux(self):
         elif ratio <= 1.30:
             lux = 0.00146 * ch0 - 0.00112 * ch1
         else:
-            lux = 0
+            lux = 0.
         # Pretty sure the floating point math formula on pg. 23 of datasheet
         # is based on 16x gain and 402ms integration time. Need to scale
         # result for other settings.