[ECI-420] Use Bearer authorization for requests

[tiannaavery]

Closes #25

Fulfills request to use Bearer authorization. I successfully tested this locally.

Currently, the way the documentation says an access token can be used is not valid. The new API definition is as follows.

$client = new \Drip\Client(["account_id" => "YOUR_ACCOUNT_ID", "access_token" => "YOUR_ACCESS_TOKEN"]);
$client = new \Drip\Client(["account_id" => "YOUR_ACCOUNT_ID", "api_key" => "YOUR_API_KEY"]);

The intention is to continue to support the previous client definition and maybe remove it eventually.

$client = new \Drip\Client("YOUR_API_KEY", "YOUR_ACCOUNT_ID");

I also changed references in the code from "api token" to "api key" just for clarity and consistency.

Very open for suggestions on how this could be cleaned up or made better.

[atongen]

👍

[will-in-wi]

This looks great!