Refactor parameters

Hey,

I don't know if this library is still maintained, but I made some changes. If this isn't maintained, I'll just use my own fork..

I was wondering about the way you use the parameters. As I understand in omnipay, you have (see https://github.com/omnipay/targetpay/pull/1#issuecomment-39220023):
transactionId = the id created by the developer (primary db key or something)
transactionReference = created by the payment provider, to keep track of the transaction.

And according to the documentation, the entranceCode is to keep track of the transaction, and that is also returned in the responses (but the purchaseId also). So it would be more logical to use that as the transactionId, right?

And the trxid returned in the responses is different then the purchaseId or entrancecode, so should be the transaction reference. And this can be gotten from the request after the return.

- entrancecode = transactionId
- trxid = transactionReference
- purchaseid = purchaseId
- camelCase the omnipay parameters
- merchantKey is a default parameter (always used)
- get the transactionReference from the query

What do you think? Not entirely sure about the purchaseId/entranceCode, but it seems entranceCode is more used for the purpose of transactionId, while purchaseIs is more for tracking it in the transaction (although probably most of the time, they are just the same..)