Feature/pass headers

[scottusrobus]

Adds header support, along the same lines as @ksheurs added to the Ruby Client. 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' => 'customer@email.com',
  'product' => 'https://api.vhx.tv/products/1'
), array(
  'VHX-Client-IP' => '0.0.0.0'
));

@ksheurs 👀

[ksheurs]

yay headers! nice @scottusrobus !