Typo fixes, mainly misuse of [it’s][oatmeal-1]/[its][oatmeal-2].

[oatmeal-1]: http://s3.amazonaws.com/theoatmeal-img/comics/apostrophe/4.png
[oatmeal-2]: http://s3.amazonaws.com/theoatmeal-img/comics/apostrophe/5.png