Make API client and buildURL method public

[Flynsarmy]

I'm needing to make some requests that you haven't implemented yet - namely posting a new status. Doing so is relatively easy with the following code:

$response = $api->client->request(
    "POST",
    $api->buildURL('statuses', []),
    [
        'headers' => [
            'Authorization' => "Bearer {$api->accessToken}",
        ],
        'form_params' => [
            'status' => "Status test",
        ]
    ]
);

However the client property and buildURL method of Fundevogel\Mastodon\Api needs to be made public for this to be possible. You could also provide a public getClient() method if you're worried about the property needing to be read only.

I understand your API once implemented will be much cleaner than what I have above, but it's nice to be able to make custom requests without needing to wait for the implementation.