Quoting gcc-4.8.2

> espurna/sensors/BaseAnalogEmonSensor.h:19:7: note: 'BaseAnalogEmonSensor' is not literal because:
> class BaseAnalogEmonSensor : public BaseEmonSensor {
>       ^
> espurna/sensors/BaseAnalogEmonSensor.h:19:7: note:   'BaseAnalogEmonSensor' has a non-trivial destructor

- fix dubious pinMode(0, INPUT), which does not work with esp8266 as A0 != GPIO0
- I2C sensor locking code can now exist separately from the base class
- EmonSensor -> BaseAnalogEmonSensor
- move AnalogEmon-specific methods from BaseEmonSensor to BaseAnalogEmonSensor
- clean-up protected member usage across the base, force the class to implement certain methods instead
- more code reuse between sensor versions
- drop local magnitude indexes & shared arrays of values in favour of separate
class instances, refactor ADS1X115 to the new model
- (techically) allow ADS1X115 code to be used with more than one i2c address
by having a shared port instance