isNull, isUndefined convenience functions

[jmagaram]

I was writing some code checking for null and undefined and found it awkward. Originally I tried %raw which worked but wasn't pretty. I tried a bunch of Nullable.toOption and then checked for None and that was a mess. Then I realized I could use the == operator, but that also wasn't pretty. There was the uncertainty of do I use == or ===. In JavaScript null==undefined I think. The worst is if you are checking if something is null or undefined...

Nullable.make(obj)==Nullable.null || Nullable.make(obj)==Nullable.undefined
Nullable.make(obj).isNullOrUndefined

So here are some convenience functions like Nullable.isNull and Nullable.isUndefined that are analogous to Option.isNone and Option.isSome that make the code more readable. Includes tests and docs.