support different rounding modes

[patch]

As per the CLDR spec (see below), default rounding is half-even. There is no current way to change the rounding mode. We use Math::BigFloat, which supports the following modes: even, odd, +inf, -inf, zero, trunc, common. Let's add a rounding_mode attribute and decide if we should use the same modes and names as Math::BigFloat.

An implementation may allow the specification of a rounding mode to determine how values are rounded. In the absence of such choices, the default is to round "half-even", as described in IEEE arithmetic. That is, it rounds towards the "nearest neighbor" unless both neighbors are equidistant, in which case, it rounds towards the even neighbor. Behaves as for round "half-up" if the digit to the left of the discarded fraction is odd; behaves as for round "half-down" if it's even. Note that this is the rounding mode that minimizes cumulative error when applied repeatedly over a sequence of calculations.