Add SEPA Creditor identifier (AT-02)

Sepa Creditor identifier is a code used on SEPA Files to identify the persons/bussness involved in the transaction. 

All the details of the implementation can be found on: 

https://encrypted.google.com/url?sa=t&rct=j&q=sepa%20at%2002&source=web&cd=1&cad=rja&sqi=2&ved=0CC8QFjAA&url=http%3A%2F%2Fwww.europeanpaymentscouncil.eu%2Fknowledge_bank_download.cfm%3Ffile%3DEPC002-09%2520e-Mandate%2520Service%2520IG%2520V4.0%2520Approved.pdf&ei=F_vxUpmvIrLT7AaEpIAI&usg=AFQjCNH4wunyM7J0-udLdSOFz17fQrBmfg

Page 8, section 1.5.2