Issue #34 Add isValid assertion

[ystarangl]

Resolves #34

Added new isValid assertion. Updated docs.

The only thing to note here is that there is no expected value because we're expecting anything except invalid. So I'm passing non-invalid string there.

Sample test case:

m.it("in the test case", sub()
    m.assert.isInvalid({}, "should be invalid")
    m.assert.isValid(invalid, "should be valid")
end sub)

When isInvalid not commented out:

When isValid not commented out: