Merge pull request #73 from tzik/virt

fix virt-specifier
Merge pull request #74 from tzik/typo

typo fix
Merge pull request #75 from tzik/pre

fix <pre>
Merge pull request #76 from tzik/more_pre

fix yet other <pre>s
Merge pull request #77 from tzik/fix

fix
Merge pull request #78 from tzik/constructor

fix condition of implicit default constructor
explain that virtual function need not be defined if it's not called.
Merge branch 'gh-pages' of https://github.com/EzoeRyou/cpp-book into gh-pages
fix implicit default constructor description