Round promocode values

Prevents 'The link you have used to enter the PayPal system contains an
incorrectly formatted item amount' messages when applying promo codes
that return fractional amounts (e.g. 7.9992 => 7.99)
Ooops, underwear is showing.
Fix wrong table prefix (thanks Krisztián)

Also improve upgrade process so that, in the (hopefully now unlikely)
event the table exists when the shop is installed, there aren't 50
errors thrown.
Merge pull request #1 from Bloke/master

Test updating from block