Implemented timestamp + dbtimestamp row version

Have added support for the last 2 scenarios mentioned in http://ayende.com/blog/3946/nhibernate-mapping-concurrency

So now, if a property is of DateTime, we get a timestamp column and if a property is byte[] we get dbtimestamp

I also fixes a bug where the generated was set to always and for int and datetime it should be never.

This is related to #18 

@andreasohlund and @johannesg can you please review?