Add an id attribute to references.
Expose a `versionOf` attribute.

Make `bibref.get` ship the canonical reference when
specific versions are requested.
Add a reverse lookup table and expose it the HTTP API.

This enables finding specs through URLs.
Fix broken test suite.
Update README accordingly.
Add examples to README.
Merge pull request #148 from tobie/reverse-lookup

Adds id and versionOf attributes; Create an new HTTP API for looking up specs with a URL