Remove testing and unused code. Use more constants

* Removed get_data() & test_request() methods
* Alias \eBay_API\Credentials as Credentials
* Class constants for: PHOTODISPLAY, SANDBOX|PRODUCTION_REQUEST
getCategoryName() now checks results

Save query results as a variable.
Check that results are object and have name
Return the category name if so, else return null
No static var for $db in package_info() method

Use PDO::load() instead.
Correct settings sandbox/production envirmentment call requests
Replace getCategoryName() method with getCategory()

Use StoreCategoryID instead of CategoryID as well
Return object containing Name, Code, & CategoryID
Remove VerifyAddItem call option

Just use as AddItem.
Maybe copy class as VerifyAddItem, but this is to only do AddItem
requests