Temperature regulator

@verdeimpacat
It is unnecessary to call

//soft reset is required before use of HTU21D
htu.softReset();

I put this call to the constructor of HTU21D which will be called during "require('HTU21D').connect(…);". Soft Reset is recommended in the data sheet, so nobody can forget it.

function HTU21D( i2c ) {
  …
  this.softReset(); // Soft reset is recommended at start according to the datasheet.
}