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.
Update the release note to include the addition of the IsNegativeOrZero() check.
Mark IsGreaterThan() obsolete and introduce IsStrictlyGreaterThan() instead.
Mark IsLessThan() obsolete and introduce IsStrictlyLessThan() instead.