chore(.gitignore): update
feat: add aux files for PDF generation
feat(*.org): move HTML-related headers to header/header.html.org
feat(Makefile): add PDF generation
feat(merge_chapters.sh): add merge_chapters.sh
chore(.travis.yml): install required package for pdf generation
feat(elisp/org-pdf-export.el): remove "inputenc" from default packages as it clashes with xelatex
feat(elisp/ob-lean.el): remove unused file