Review #2

This PL contains the result of my review

* a bunch of little typos fixed
* some sentences reformulated in a (hopefully) lighter form
* the reference to James Shore replaced with a reference to Kent Beck (Shore is not the author of the idea of XP coach, Beck is)
* a reference to Race Condition in the sentence about FlexCoin
* the name FlexCoin is revealed after the explanation of  the Race Condition