@beamjet Checkout form html, checkout data validation

You can see the specs for how to use this lib. Let me know if you have any questions.

Current specs:

```
  #initialization
    ✓ has default attribute values for AllPay testing environment 
    ✓ overrides default attribute values with options passed in 

  #genCheckMacValue
    ✓ generates correct Check Mac Value 

  #createFormHtml
    ✓ generates correct HTML form 

  #validateCheckout
    required fields
      ✓ requires MerchantID 
      ✓ requires MerchantID to be no more than 10 chars 
      - requires MerchantTradeNo
      - requires MerchantTradeNo to be no more than 200 chars
      - requires PaymentType
      - requires PaymentType to be "aio"
      - requires TotalAmount
      - requires TradeDesc
      - requires TradeDesc to be no more than 200 chars
      - requires ItemName
      - requires ItemName to be no more than 200 chars
      - requires ReturnURL
      - requires ReturnURL to be no more than 200 chars
      - requires ChoosePayment
      - requires ChoosePayment to be one of the supported payment moethods
    payment methods
      ALL
        ✓ keeps all the required fields 
        ✓ discards invalid options 
        ✓ keeps all options except options of credit card 
        ✓ discards options of ignored payment methods 
        ✓ keeps overlapped fields if not all of the payment methods are ignored 
        ✓ discards ClientRedirectURL option if it is blank 
      Credit
        ✓ keeps all the required fields 
        ✓ returns errors if passed in both installment and periodic options 
        ✓ keeps only installment credit card and ALL options besides required fields 
        ✓ keeps only periodic credit card and ALL options besides required fields 
      WebATM
        ✓ keeps all the required fields 
        ✓ keeps only ALL options besides required fields 
      ATM
        ✓ keeps all the required fields 
        ✓ keeps only ATM and ALL options besides required fields and discards IgnorePayment 
        ✓ discards ClientRedirectURL option if it is blank 
      CVS
        ✓ keeps all the required fields 
        ✓ keeps only CVS and ALL options besides required fields and discards IgnorePayment 
        ✓ discards ClientRedirectURL option if it is blank 
      BARCODE
        ✓ keeps all the required fields 
        ✓ keeps only BARCODE and ALL options besides required fields and discards IgnorePayment 
        ✓ discards ClientRedirectURL option if it is blank 
      unsupported payment method
        ✓ returns unsupported errors 
```