artifact binding support

[amdonov]

Relates to #40

This integrates Artifact Binding support. I tried to make as few changes as possible. In my tests I was able to switch back and forth between post and artifact binding without issue.

I did have to write a different example client because Okta seems to always respond with a post binding response regardless of the request made. I didn't include my example in the commit because it involved having a working IdP setup and probably would be of limited use to others.

SP creation in it looks like the following:

sp := &saml2.SAMLServiceProvider{
IdentityProviderSSOURL: metadata.IDPSSODescriptor.SingleSignOnServices[0].Location,
IdentityProviderArtifactResolutionServiceURL: metadata.IDPSSODescriptor.ArtifactResolutionService.Location,
HTTPClient: client,
IdentityProviderIssuer: metadata.EntityID,
ServiceProviderIssuer: "http://example.com/saml/acs/example",
AssertionConsumerServiceURL: "http://localhost:8080/v1/_saml_callback",
SignAuthnRequests: true,
AudienceURI: "http://example.com/saml/acs/example",
IDPCertificateStore: &certStore,
SPKeyStore: keyStore,
RequestedBinding: saml2.BindingHttpArtifact,
}

IdentityProviderArtifactResolutionServiceURL, RequestedBinding, and HTTPClient are the new fields. None are required if clients want to stick with POST binding.

On an unrelated note, I had to change BuildAuthURL to call BuildAuthURLRedirect rather than BuildAuthURLFromDocument to get valid requests for my IdP.