extract generated id by column name

See issue https://github.com/brianm/jdbi/issues/113

I added a new parameter "columnName" to the annotation GetGeneratedKeys.
This allows us to extract any type of generated key by column name from the returned result set.

I also added a test case TestGetGeneratedKeysPostgres which demonstrates how the parameter works.
It's corrently ignored because I made it for postgresql but I'll come up with a H2 equivalent soon.