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
Merge pull request #276 from mojavelinux/default-stylesheet

resolves #76 add a default stylesheet
resolves #257 add AsciiDoc config to document compatibility
resolves #244 set attributes based on safe mode
update version to 0.1.2, set release date
fix bug resolving custom stylesheet, add tests
update year in copyright
update metadata
update description
fix race condition w/ attribute override; disable embedding css in secure mode
switch readme to .adoc extension
ensure a trailing endline is output to stream
add warning if stylesheet to embed cannot be read
update license owners; update manpage
update gemspec to reflect readme rename
update version for candidate release
Release 0.1.2.cr
Release 0.1.2.cr
resolves #280 trailing comma shouldn't be included in link
update version to 0.1.2