[tut/chap2] adjusted prose describing scope and model etc

- adjusted prose describing scope and model
- Prefix annotation class name occurrences with @ for better
readability (and it more std).
- cleaned up some whitespace in code snippets
Merge pull request #15 from chalin/patch-15-adj-scope-and-model-defn-ch2

[tut/chap2] adjusted prose describing scope and model etc