resolves #76 add a default stylesheet

- add a default stylesheet, asciidoctor.css, to stylesheets directory
- add copycss attribute to control copying default stylesheet to outdir
- add linkcss attribute to control stylesheet embedding
- use default stylesheet by default w/ linkcss enabled
- rewrite path logic to be more secure, deterministic and OS independent
- switch all path resolution to use new path resolver
- add path resolver tests
- refactor cli invoker to use render API
- add timing monitor to load and render APIs
- fix image path resolution to be secure
- add tests for embedded attribute