Add specification documentation

Still incomplete, but review may start on the parts that are in there.

Fingerprint definition if still missing

ping @jlhawn 
I'm not sure I understand this sentence, how is a certificate "self-signed by a key managed by libtrust"? Are there two private keys here, or just one? If the latter, I'd suggest breaking the sentence into two; specify that the cert is self-signed, and then describe how the assosciated pk is managed.