Mostly grammar and consistency changes

Collected some language fixes when reading the document. There should be no change in meaning anywhere, but of course feel free to ignore parts that do not "feel" right.

The "Consistency" fixes standardizes on the following forms:

- Windows
- Unix
- OS X
- GNU
- Vim
- Git
- Google

In case of uncertainty ("Unix or UNIX?", mostly), I went with what Wikipedia chose.