SSL/TLS certificates have an optional extension listing alternative

names or IP addresses that the certificate may represent. This change

implements access to these through a new subject_alternative_names

property, which returns a list of string values for all DNS and IP

addresses listed in the cert.
Callers can use this to validate the contents of a cert against other

hostnames. Note that certificates will not be indexed by these names,

just the primary hostname associated with the Certificate object

(which will usually be the host that was accessed that served the

certificate). For the time being, that makes this more useful as a way

to take an existing certificate and compare it prior to indexing under

the primary hostname.

Unit tests passed.