Adds header support. Headers can be passed as an array via the last param (either the second or third param depending on the method).
Example:
$customer = \VHX\Customers::create(array(
'name' => 'First Last',
'email' => '[email protected]',
'product' => 'https://api.vhx.tv/products/1'
), array(
'VHX-Client-IP' => '0.0.0.0'
));