Add a new OAuth method utilizing JWTs

[dbhynds]

This PR adds a new OAuth method using JWTs as described here:
https://help.salesforce.com/articleView?id=remoteaccess_oauth_jwt_flow.htm&type=5

Per the Salesforce documentation, this authentication method does not refresh tokens. As such, the refresh() method just calls authenticate() to generate a new token.

I manually tested this with an app that is set up for JWT auth to verify it works. I also wrote some basic tests based on examples set in other Spec files. Because JWT assertions incorporate an expiration timestamp, I imported the Carbon library and used Carbon::setTestNow() to freeze time so the generated assertions won't change while running tests.

[omniphx]

Hey @dbhynds this looks awesome! I know a lot of people have been requesting JWT. Looks like there is one small conflict to resolve from a recent merge

[dbhynds]

@omniphx Merge conflicts fixed. Let me know if there's anything else.