/parser should now behave exactly according to the specs

Implements some parts of https://trello.com/c/N7TSt5CB/1612-api-for-parsing-blueprint-from-markdown-to-ast.

- `js-yaml` has better output than `json2yaml` and it's more robust (...but it doesn't support dumping of arbitrary objects - https://github.com/nodeca/js-yaml/issues/132)
- Content negotiation returns YAML in case client wants it, otherwise it returns JSON AST.
- Pinned `express` so it's upgraded during deployment.