Use API key if provided

[lancergr]

Description
Use API key if provided
Added PaymentsRequest constants and helpers

Tested scenarios
Payments and Modifications

[coveralls]

Coverage decreased (-0.1%) to 27.075% when pulling 7537f42 on apikey into da99e9d on develop.

[rkewlani]

It would be good to add log statement in catch before returning null.

[rkewlani]

Also, we can skip return statement in catch and replace it like :
Date date;
SimpleDateFormat monthYear = new SimpleDateFormat("M/yyyy");
try {
date = monthYear.parse(expiryDate);
} catch (ParseException e) {
//log statment here
}
return date;
}