Added dotpay CHK parameter support and Symfony version requirments.

1 changed:
Symfony version requirments change to <2.4

1 added:
Added support of CHK parameter. CHK parameter is used by dotpay do sing payment request. It is used to avoid generation of payments by anyone. The problem is that anyone can use this form https://ssl.dotpay.pl/payment/index.php or send false request to doypay about new payment. In this case you can generate many 0,01 transaction for witch the DotPay customer have to pay fee. The CHK parameter can be enabled by contact dotpay support tech@dotpay.pl. Email must be send from email associated with dotpay account.