fixed spelling mistakes, grammar fixes etc.

Mainly fixed some spelling mistakes etc. Also I changed the use of the word "data". According to this article: http://www.theguardian.com/news/datablog/2010/jul/16/data-plural-singular it's very uncommon to use "data" with a plural verb. Also it reads really strange and "too formal". So I changed everything from "the data are" to "the data is"...