Renamed Cheshvan to Marcheshvan

[CompuGeniusCode]

Besides wanting the proper name, I also see that you mentioned wanting to fix it here: https://github.com/KosherJava/zmanim/pull/131

Pretty sure I got all the places it was spelled incorrectly. I also renamed function names, so you may want to only accept the string changes now and wait for the functions in a new breaking release, but that's up to you.