BACKWARDS COMPATIBILITY NOTICE:
* Requires Vault 0.1.2 or later for `X-Vault-Token` header
* `auth_token` method removed in favor of `token` property
* `read` method no longer raises `hvac.exceptions.InvalidPath` on nonexistent paths
IMPROVEMENTS:
* Tolerate falsey URL in client constructor
* Add ability to auth without changing to new token
* Add `is_authenticated` convenience method
* Return `None` when reading nonexistent path