Adding a library

[Jorropo]

I follow contributing guidelines to add this lib :
https://github.com/Jorropo/ds3231
This is a reborn of the old ds3231 made by Rodan.

[cmaglie]

done!

[njh]

@Jorropo It looks like this repo has now been deleted. Any information about why?

[Jorropo]

@njh I was not maintaining it and there is better alternative. (the original is still available here https://github.com/rodan/ds3231/ with the same api).