database-independent column quoting

I don't think the parser's current `to_sql` method will work with postgres, which escapes column names with double quotes rather than backticks.

I've update the method to use `ActiveRecord::ConnectionAdapters::Quoting#quote_column_name` to do the quoting. This means passing a copy of the DB connection adapter into the to_sql method, which I think is fine (this method should only work with ActiveRecord anyway, right?)

Might be worth setting travis up to test against multiple database backends, which I have not attempted here.