Addressed an issue casued by NSRangeException : Range or index out of bo...

...unds in the columns method of ZIMOrmModel

### Changes:

* A very small change to how the `columns` method of `ZIMOrmModel` was creating an `NSRange` for use in `substringWithRange:`. Converted `substringWithRange:` to `substringFromRange:` which will substring from the index given to the end of the string.

This fix resolves this issue: https://github.com/ziminji/objective-c-sql-query-builder/issues/20