Match PremiumMoney interface to Money
Round premium payment amounts using bankers' rounding
Implement bankers' rounding manually

The Ruby 1.9.2 BigDecimal implementation contains a bug affecting rounding half to even:
http://bugs.ruby-lang.org/issues/show/3803
Merge pull request #123 from alphagov/bankers_rounding

Use bankers' rounding on premium payment amounts