Implement bendable score of BigDecimal type

This pull request implements ```BigDecimal``` bendable score.

The alrogithms used are a mix of those from ```BendableScore``` and ```HardSoftBigDecimalScore```. The tests have been adapted from those as well, with two exceptions:

- The ```power()``` tests have been ```@Ignore```d here. The whole problem of "BigDecimal to the power of BigDecimal" deserves a solution, and a skipped test will always remind us of that.
- The ```multiply()``` tests have been removed. The originals were testing for int result of double multiplication, and that is simply not applicable here.

All the tests are passing.