Implement JWT authentication #118

[aisipos]

Support OAuth JWT grant authentication, as defined here:
https://developers.docusign.com/platform/auth/jwt/jwt-get-token/

Client instances will need to be passed several new configuration variables to support this:

• private_key (in textual PEM format)
• user_id
• integration_key
• audience
• base_url

If these values are not provided, client instances will fall back to the prior behavior, of using either OAuth2 password grant or username and password authentication.

If these values are provided, client instances for each request will request a new access token using the new _get_access_token method, then use that when doing the actual Docusign REST API request.