Unit price tax calculation bug

[karashiiro]

Currently, both the unit price and the total are presented in the API with the 5% sales tax precalculated. However, the formula for calculating the new values (ceil(x * 1.05)) loses information, which can, for some values, cause the taxed unit price times the quantity to not equal the taxed total price.

Fixing this would require changing the interface of the current API versions to allow the unit price to be a floating-point number, which would break existing projects written in statically-typed languages. This should therefore be fixed in the V3 API.