Correct inverted uses of "only"

[de0u]

After seeing a few instances of "only" in the wrong place I went through the whole book looking at each usage of "only". I tried to make the smallest possible change in each case, i.e., I tried to avoid non-essential re-wording. I realize this is a big commit but I think I restricted it to one topic.

[marijnh]

Wow, that's a lot of them, but indeed, I guess the changes are an improvement. Merged as https://github.com/marijnh/Eloquent-JavaScript/commit/d0a740c0816c1bb9bd862b5166bbb17e1320aa59