implement "dry run" flag

[DavidLeoni]

We should support some "dry-run" import functionality.

A realistic "dry run" is currently not possibile because system is not fully transactional: UBY performs many commits during import, and even if we change it to do just one commit I don't even know if it is possibile at all to run a huge transaction without making Hibernate explode. For sure we can't do many small transactions inside a bigger one, because nor Hibernate nor H2 support nested transactions.

So we can at least simulate import to the point when db is checked prior actually running UBY import.

[DavidLeoni]

Note dry run just goes until third step of validation includeed where it checks references against the db
It does't try the ideal solution which would be doing a a transaction with a successive rollback because of #38