Improvements and fixes by dusank

I am currently in the process of reviewing SBT in Action and will be pushing any improvements and/or fixes onto this branch.

=Dusan
I understand this change, but it's not the case that we've described `ThisBuild` very well in chapter 4.  It isn't until later that the full scope hierarchy is discussed, so this ne I don't think belongs in chapter 4 right now.