Allow hostname verifier and socket factory for http

[AaronAtDuo]

@mmoayyed had asked for this in https://github.com/duosecurity/duo_client_java/pull/29

I took a side trip to implement a Builder to make it easier, so now I need to actually add the new functionality...

[mmoayyed]

As a side note, it would also be great if the CertificatePinner could be injected via the builder, instead of :

CertificatePinner pinner = Util.createPinner(this.host, DEFAULT_CA_CERTS);