Added Exists() method as per issue #2 on likexian/simplejson

Once again, added the new Exists method as defined in [issue #2](https://github.com/likexian/simplejson/issues/2). 

Also added test cases for the new method. As per my last pull request, there we're so many updated files because my text editor automatically applies gofmt, so all the spaces were replaced with tabs (and other formatting)