Add assertions for database structure

I need assertions for checking database structure.
E.g. I want to check if specific table exists in database.

Proposed assertions:
`assertThat(table).exists();`
or
`assertThat(source).tableExists("name");`

Additionally it would be nice to have convinience method isEmpty() for
`assertThat(table).hasNumberOfRows(0);`

JDBC code to check if table exists ()

```
public static boolean hasTable(DataSource ds, String name) {
boolean exists = false;

try(Connection c = ds.getConnection();
//table names usually stored in database uppercased
ResultSet rs = c.getMetaData().getTables(null, null, name.toUpperCase(), null)) {

if (rs.next()) {
exists = true; // table exists
}
} catch (Exception e) {
e.printStackTrace();
}

return exists;
}
```

Or just do some operation and catch jdbc exception: table not exists. Below assertion partially does the trick, but is not elegant.
`assertThat(table).hasNumberOfRows(0);`