Add option isbn for books
Add gitbook init command to "how to use" section

Highlight that Gitbook can generate the files and directories for a book from its SUMMARY.md file.
Merge pull request #516 from jr0cket/patch-1

Add gitbook init command to "how to use" section
Merge branch 'master' of github.com:GitbookIO/gitbook

Conflicts:
	lib/generate/config.js