Added support for Rain Gauge

Added mapping for Rain measurement in SENSOR_PROPERTIES and updated
usage documentation.

Running samples/printAllLastData outputs:
```
Rain Gauge
                     rf_status : 60
                    battery_vp : 6402
                          When : 06:39:45
                          Rain : 0
Indoor
                         Noise : 34
                   Temperature : 21.2
                          When : 06:39:53
                      Humidity : 31
                      Pressure : 1035
                           Co2 : 542
Indoor 2
                           Co2 : 468
                     rf_status : 63
                   Temperature : 23
                          When : 06:39:38
                      Humidity : 27
                    battery_vp : 5882
Outdoor
                     rf_status : 59
                    battery_vp : 6132
                          When : 06:39:51
                   Temperature : 0.3
                      Humidity : 61
```
Added real_time argument to getMeasure()

By default, for scales other than max, the timestamp of values is
offset by +scale/2. For example, for a scale of 1day the times amps
will be at noon. The real_time causes the timestamp to be the beginning
of the period. Setting realtime to True keeps the timestamp from
potentially exceeding date_end.
Modify lastData Rain comment

Added information about unit-of-measure for Rain in lastData method.
Merge pull request #1 from asphotographics/master

Added support for Rain Gauge