Reapplying realex gateway addition changes

@davidjrice fyi -- I reapplied your changes against most recent master. 
@davidjrice Can I recommend that the Billing and Shipping Code are formatted in the following way:
Billing Code = 'digits from postcode|digits from first line of address'
So for example an address of

Flat 456
House 789
E12 3QJ

Will result in a billing code of "123|456"
Formatting the data this way will make it compatible for AVS checks for UK based customers. So just the digits from the first line of the address and postcode must be extracted and concatenated with a '|'.