fix db:dump --single-transaction switch

The readme says `db:dump` has an option:

>` --no-single-transaction`
> Do not use single-transaction (not recommended, this is blocking)

https://github.com/netz98/n98-magerun/blob/develop/src/N98/Magento/Command/Database/DumpCommand.php#L193
Current behaviour is the inverse of this, if `--no-single-transaction` is specified the mysqldump actually gets the argument `--single-transaction`

`--single-transaction` actually blocks and only applies to InnoDB tables: http://dev.mysql.com/doc/refman/5.1/en/mysqldump.html#option_mysqldump_single-transaction

I added the `--quick` argument as it's recommended for large tables when using `--single-transaction`

To be clear this now puts `--single-transaction --quick` in the default `$dumpOptions`. This matches my interpretation of what it says in the README and help command.
I am happy to refactor this (including help and README) to make this command opt-in if that was your intent.