Add support for reading battery voltage

[reedstrm]

Recently purchased a number of these sensors. Found your blog post about this code, and noted the mystery about the last two bytes. Some bash code in a repo by @moras-p https://github.com/moras-p/mijia indicates these are little endian battery voltage * 1000, so I added that to your code. Seems you're probably not using this repo past the intiial experiment, but figured I'd leave this here for anyone else who stumbles across it.