Incorporate solutions into the book

[ro31337]

There are solutions for some chapters (for every other exercise or so there is a solution), see here https://github.com/ro31337/rubyisforfun/blob/master/manuscript/solutions.md

We need to create a section in the book and incorporate solutions into the PDF.

It's OK (and even better) if you roll out it partially and do work in incremental steps, incorporate a couple of solutions and introduce a PR. The first PR should also modify the book.txt - https://github.com/ro31337/rubyisforfun/blob/master/manuscript/Book.txt - this is an index file for LeanPub. You might want to add solutions txt file right to the end.