adapt description
added spec-helper to easily cover many test cases
added ifNotNumber()
rewrite tests using new helper
added ifNotString()
added a not() mode
remove ifNotEmpty(), ifNotNull(), ifNotUndefined() and ifNotEquals()
change ifNotNumber() to ifNumber()
change ifNotNumeric() to ifNumeric()
change ifNotString() to ifString()
rename registerRejector() to createRejector()
added tests for not()
added ifBoolean()
added ifArray()