Better YAML support

- Parse any yaml with PyYAML
- Add builtin yaml keyword references for specifying references
- Add builtin yaml keyword basepath for specifying base path for image
sources and such
- yaml title keyword changes html title
- All other keywords are put into a meta object to create meta html tags
- Actually retrieve meta variables from Python Markdown “meta”
extension and merge with yaml meta (yaml meta overrides “meta” extension
- Fix for ST2 super reload problem
- Fix for ST2 choking on using pkutil on “yaml.lib3”
- Add test markdown for testing features