Spell check and typos

A few things are debatable, if you really care about shed colors. I went for American spelling (organi[-s]{+z}ation, favo[-u-]r), and replaced amongst with among, the latter being more modern and common. I also expanded a few abbreviations (BTW, GSoC). The rest are actual typos.
Merge pull request #68 from RailsGirlsBerlin/arne_spellcheck

Spell check and typos