Implement Assert.isNumeric()

Hey guys,

On several occasions I've been placed in situations where values that I was testing could be either numbers or strings representing numbers. I know I could use `areEqual()` but this is intended for when you don't know the value in advance.

The reason why I'm rejecting NaN and the like is because `parseFloat('Infinity')` wouldn't return a number (well yeah but...). So I'd rather have the assertion works similarly for both numbers and strings.

And also I don't like NaN...

Hope you like it.

Cheers,