Various Updates

Credentials: fetch method is public
eBay_API_Call: update call for AddFixedPriceItem
Added AddFixedPriceItem as a request
New static method \eBay_API\Credentials::token

Similar to fetch method, except that it returns only [
	'eBayAuthToken' => $token
]

fetch is protected again. token is public
Use Credentials::token for RequesterCredentials

Since it returns ['eBayAuthToken' => $token], no need to add
'eBayAuthToken' element.
Update $call_list for AddItem

AddItemRequest should be AddItem. Had them in reverse.
New request class AddItem

Another work in progress. Extends eBay_API_Call for AddItem requests