Use library of congress namespace for crypto hash terms

As per #7 and ni-/notebooks#14, this is a proposal to use the namespace for crypto hash functions defined by the library of congress. This provides:

- terms for specific hash functions rather than just "sha" (e.g., sha1, sha256, sha512)
- content negotiation to request html or rdf representations of the terms

Also updated `crypto:sha` to be `crypto:sha512`, but could be any version.