add .isZero() comparison

[alensiljak]

Hi,

Checking whether an amount is zero, negative, or positive is quite a common usage scenario and adding helper methods would make this check much easier.
Currently we have to create a zero object and compare the actual value with this.
JavaMoney has some great comparators, like .isZero(), isZeroOrNegative(), etc. (https://github.com/JavaMoney/jsr354-ri/blob/master/src/main/java/org/javamoney/moneta/FastMoney.java)
Would be great if Money type had something similar built in.

i.e.

@Override
    public boolean isZero() {
        return this.number == 0L;
    }

    @Override
    public boolean isPositive() {
        return this.number > 0L;
    }

    @Override
    public boolean isPositiveOrZero() {
        return this.number >= 0L;
    }

    @Override
    public boolean isNegative() {
        return this.number < 0L;
    }

    @Override
    public boolean isNegativeOrZero() {
        return this.number <= 0L;
    }

BigDecimal implementation is using .signum().
https://github.com/JavaMoney/jsr354-ri/blob/master/src/main/java/org/javamoney/moneta/Money.java

[ratcashdev]

You may want to consider using SIGNUM.
More details here: alensiljak@97dabbf#commitcomment-29308125