Add option to adjust issue instant to account for clock skew

[zachelrath]

Clock skew in the assertion consumer can cause assertions created using node-saml to be invalidated due to the NotBefore SAML condition being in the future according to the assertion consumer's clock. The lifetimeInSeconds option accounts for some clock skew in one direction, but does not account for the assertion consumer's clock being ahead of the assertion producer.

I have encountered this issue trying to use node-saml to create assertions for consumption by Salesforce. Backing up the issue instant by 60 seconds or so accommodates for the clock skew between the systems from which I am producing assertions, and Salesforce's servers.

If we had an option such as issueInstantSkewInSeconds, we could subtract this from the issue instant . Coupled with lifetimeInSeconds, we can effectively account for clock skew.