/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.
`res.json` uses `res.send` internally. See https://github.com/visionmedia/express/issues/2238 for explanation of my (in fact, @kuba-kubula came with the hint) solution, it's "approved" by express devs.

I thought I can test the whole API by dredd, which is not yet the case. So I am testing manually now. I'd love to add proper tests - maybe tomorrow after `/compose` is ready.