Update .gitignore with build folder + add built epub

Exclude build folder: Book
Add built epub: Book.epub
Update CodeKit config files
Add files to build frontmatter and backmatter
Refactor header and close .kit files

Split header and close .kit files into separate versions for flowing
and fixed layouts.
Add metadata for chapter headings
Add metadata for optional contributors
Move variable options and add default values

List options for certain variables in their own comment and add a
default value so that built epub will validate even if no variables
have been changed from their defaults.
Update default values for content .kit files
Update toc.kit to include heading variables
Add base layout css and separate mobi .scss file
Import layout-mobi.scss to stylesheet
Update package.opf with frontmatter and backmatter