round half-even with rounding increment

[patch]

By default we use Math::BigFloat for rounding and round in half-even mode. If a rounding increment greater than 1 is provided in the pattern or via the rounding_increment attribute, we instead use Math::Round::nearest because it supports rounding increments; however, it does not support half-even rounding, which I believe we should be performing along with rounding increments. We need to investigate alternatives and possibly ask for clarification on the CLDR mailing list.

See also issue #30.