Change type of Column.setIndex() from int to Integer

As an int, you can't null it out, and it's a disallowed field for POSTs,
so there is no way to POST a new column using an existing column that
you have fetched.  This change allows you to set index to null.
Merge pull request #1 from kevin-r-tao/master

Change type of Column.setIndex() from int to Integer