Deprecate submit() method, use post() instead

[clue]

This changeset deprecates the submit() method and suggests using the existing post() method instead:

// old: deprecated
$browser->submit($url, $data);

// new
$browser->post($url, ['Content-Type' => 'application/x-www-form-urlencoded'], http_build_query($data));

Builds on top of #170