Add `#ifdef BH1750_DEBUG` guards around all serial print statements

[QuentinTorg]

There were already #ifdef macros wrapping some of the serial print statements in the library. This PR adds macros around the remaining Serial print statements.

The serial print statements can be very noisy. They are helpful when debugging the BH1750, but for some use cases this can be extremely overwhelming. This is particularly hard to deal with when using serial prints to debug other sensors without the BH1750 sensor connected