execute_with_columns method to allow users to retrieve also column names

[tmontanaro]

execute_with_columns method to allow users to retrieve also column names.
Maybe a better integration is possible, but it was the easiest and fastest way to do it with a compatibility with old structure

To use it:

rows, columns = sql_worker.execute_with_columns("SELECT * from tester")

[palantirtech]

Thanks for your interest in palantir/sqlite3worker, @tmontanaro! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.