/parser should now behave exactly according to the specs

Implements 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.
Gah :(

Yup, what can we do...it's just this creates basically three in-memory copies of the blueprint. Meh. 

`dredd` is for blueprint validation; it doesn't take away the responsibility for having proper test suite, including corner-case validation.