New class/trait for common eBay_API methods

Import shared methods by:
use \eBay_API\resources;
Made various API request more consistent

All request classes now have their own const CALLNAME
parent (eBay_API_CALL) will no longer us callname as an argument, but
rather use $this::CALLNAME
Make eBay_API_Call class more extensible

- Is now an abstract class. Do not use on its own.
- Moved all but construct method to Resources class/trait
- Do not set $url. Instead, use "($this->sandbox) :...?..." for $url
when
constructing parent
- Set headers using setHeaders() method from Resources rather than an
array_merge()
- Remove $call_list and instead use $this::CALLNAME . 'Request'
$environment & $sandbox are now protected variables