Add support for certificates for secure mode.

[breser]

This allows for an optional Certifying Authority certificate as well as a Client certificate and key for use with secure mode. Allowing git2consul to be used with servers that have verify_incoming turned on.

Consul client library had to be upgraded to pick up a new enough version of the node consul client library and it's dependency papi to support this.

[gotson]

would you have a ready to use binary for this PR ? I definitely need this and would be happy to test it :)

[igoratencompass]

+1 would love to have this option too and test asap.