Fixed issue #3 where atob is not defined in node-webkit

Fixed issue #3 where atob is not defined in node-webkit by first checking for its existence in case it is defined, then trying window.atob if atob is undefined.

I coded this so it is backwards compatible with whatever may have been using atob in the first place.  I make no assumptions about it being undefined in other environments and first check before using window.atob.