Draft of updates for com.amperity/vault-clj 2.x

[brandonvin]

This is currently a draft that shows what updates we might need for the vault-2.x-rewrite branch of vault-clj (https://github.com/amperity/vault-clj/tree/vault-2.x-rewrite).

This includes a couple additional improvements:

• Switch to the Java AWS SDK's default credentials provider chain and default region provider chain, since this is less surprising for most folks integrating with AWS from a Java/Clojure background.
• The login method now accepts parameters documented by Vault, though the iam_* ones are still automatically generated by the client for ease of use.