Use uc_price() instead of uc_currency_format()

Attached patch implements uc_price().

It also standardises the use of "Default price" when referring to the normal Ubercart sell_price field (rather than "Standard price" as it was called sometimes.

Updated patch - only change is to add array_filter() when checking if the uc_price_per_role_enabled variable is empty.

Hmm, always test patches no matter how trivial the change!

Attached is the correct fix for checking for the empty uc_price_per_role_enabled variable.

Thanks longwave.