Add tokens to core - address block, domain logo, domain header

Potential sprint issue for me - the address block token allows a token in pdf letters that reflects the address block formatting into pdf letters - so you don't get blank lines in the address. Issue / blocker might be that we also use the 'conditional country' token with it - that adds the country in UCASE if not the current country - this is an addressing requirement in NZ & AU for postal discounts - it I can't think how to handle this I might abort.

Also, I really want to add domain tokens for logo & header to save people having to pointlessly customise mail templates

(note I'll just close this again if I don't get it tackled at the sprint)

Here are the tokens I'll ideally add

{domain.pay_later_instructions} {domain.logo} {domain.header} {address.address_block} {address.conditional_country}