Rename IsGreaterThanZero() to IsStrictlyPositive() to follow Nicolas Morbieu's advice (https://github.com/tpierrain/NFluent/issues/120)
Rename IsLessThanZero() to IsStrictlyNegative() to follow Nicolas Morbieu's advice (https://github.com/tpierrain/NFluent/issues/120)
Add the IsPositiveOrZero() check.
Improve tests for IsPositiveOrZero().
Add the IsNegativeOrZero() check.