Adds support for JWT OAuth flow

[skostojohn]

RE Issue #66

Changes to support JWT auth flow in ex_force. Includes:

1. New function get_token_jwt to make the auth call and handle results
2. New function create_jwt_payload to create the JWT assertion payload
3. Changes to get_token to call the correct function given the grant type
4. New test helper function assert_form_body_jwt
5. New tests

The new function get_token_jwt could potentially be avoided, but there are enough differences from the other grant types that it seemed cleaner to create a new function rather than try to add this to the existing function.

I wasn't sure if I needed to make any documentation updates - I can do that if you can point me in the right direction.

As I mentioned earlier, I am a beginner with Elixir - I welcome any feedback and am happy to rework this to better suit your preferences or to make it more Elixir-idiomatic.

Thank you! Scott

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[skostojohn]

Hi @chulkilee - please let me know if you are waiting for me on any items for this PR. I am new to this, and not sure if I need to anything else to submit it for review. Thanks!