Add to .gitignore

- Sort .gitignore into some categories.

- Add artifacts for popular IDEs (Eclipse and IntelliJ IDEA).
WHITESPACE: Build.scala
Allow Answers to Reference Scalaz

I understand that the official answers may not want to reference Scalaz,
but I've found it a little useful to document more of what's out there
while fleshing out solutions to the exercises.
Add IntelliJ IDEA Plugin to SBT

I wanted to have the option to navigate through the project with
IntelliJ IDEA.  Normally I do this SBT plugin configuration under
the global ~/.sbt/plugins, but something about the way this project is
providing its own SBT bootstrap is bypassing this global configuration.

Changes like this is making the build process network-dependent.
Perhaps when I'm done with my work, I'll make it network-independent
again.
Improve Chapter 2 Exercises and Answers

- Annotate both exercises and answers with in-line comments labeling the
  exercise number to facilitate easier navigation between the code and
  main text.

- Make exercises and answers more consistent to facilitate easier
  navigation between the two projects.

- Format for consistency and clarity, including keeping lines under 80
  characters.