Cleaned up imports (unused and missing) and tweaked test (Ch.2 to 6)

 - Cleaned up imports, unused and missing (Ch.2 to 6).
 - Adjusted called to Recipe constructor in main_test (Ch.5&6).
 - Chose not to touch Chapter 7 code given that there are other pending
changes for it.
Merge pull request #46 from chalin/patch-4-mainly-import-cleanup

Cleaned up imports (unused and missing) and tweaked test (Ch.2 to 6)