Fixes #39 - NullPointerException when running "mvn liquibase:diff"

The `Table` instance obviously doesn't have a primary key set yet, since that is what we are trying to do, so set the primary key name from the `hibernatePrimaryKey` instead.
Merge pull request #40 from vetler/patch-1

Fixes #39 - NullPointerException when running "mvn liquibase:diff"