Adding jdbc_columns method that correctly uses schema information

I added the jdbc_columns method to the adapter which overrides the activerecord-jdbc-adapter jdbc_columns method. The big difference is that this method uses extract_schema_and_table to get both the schema and table names (when available). The significance of this is, if your user has access to multiple schemas with similar tables, this will isolate the schema reflection and caching (and everything else) to just the database that you specify in your connection.