Added script to format book.asciidoc for dartlang.org

Temporary changes to pubspec.
Whitespace cleanup in book.asciidoc to ensure smooth conversion to
format needed by dartlang.org.
Fixed <pre> tag generation. Other tweaks.
More changes to script to conform to dartlang format
Switched to different clef symbol that displays correctly
Merge branch 'strings'

Conflicts:
	book.asciidoc
Tweak
Merge branch 'strings'