Expose SCT parser for general use

[kpcyrd]

This allows access to the parser, as there is currently no way to get the metadata encoded inside an SCT. I've added documentation to everything that is public facing as required by #![deny(missing_docs)].

[coveralls]

Coverage decreased (-1.5%) to 89.572% when pulling a6f255a on kpcyrd:pub into 9b67a54 on ctz:master.

[cpu]

I'm thinking that it might not make sense to inflate the API surface of this crate for general purpose SCT parsing and inspection. It looks like the x509-parser crate already exposes a SignedCertificateTimestap type and a function for parsing.

Would that API serve your purpose?

[cpu]

I'm going to close this PR since it looks like x509-parser will offer what you need. Thanks!