make field nullable and table column sensitive

Hi,
It's really nice to have this framework to connect dbunit and spring, I use it a lot in my project. But I also found some issues:
1. there is no way to set one field equals null in xml dataset.
2. when the framework starts loading one table data, only the first row table columns are included, if you add one more column in later rows, it will just (silently) ignore it. 

So I add some changes to fix them:
1. add a ReplacementDataSet to replace token '[NULL]', so it's possible to set filed='[NULL]' in xml dataset.
2. set FlatXmlDataSetBuilder.columnSensing=true to make it also put the table columns added later in table medata.

I also add test methods for above changes, please have a look.

Looking forward to hear your feedback!