Option for maintaining empty values

Fix for issue #5 allows maintaining empty values using an additional
XML attribute on the Column tag called "IgnoreEmpty".  If set to true
will not update empty columns.
Better handling of NULL values

- Handles NULL values for empty columns a little better.
- Moved index icrement to the top so it doesn't need to be
  incremented before each 'continue', etc...
Unneeded import for java.sql.Types

Removed import (I added but am not using it)
Merge pull request #7 from zbateson/ignore-empty-columns

Option for maintaining empty values