v1.0.0-beta.6

Getting close to a v1 release...

• Actual documentation is developing at http://eemeli.org/yaml/
• Finalise (read: refactor) much of the public API
  • Document#contents can be set to anything
  • Add Document#resolveValue() to deeply wrap values in YAML objects
  • Move warnings from Document#errors to Document#warnings
  • Add YAML.stringify() and YAML.defaultOptions
  • Replace "stream" with "documents" in API
  • Add YAMLSemanticError class for Document#parse() errors
• String(Document) output improvements
  • tags are properly included, including prefixes
  • output is prettily folded where possible & appropriate
• Fix various corner-case syntax bugs, with thanks in particular to @ikatyang