Some tiny tweaks to db module.

Was about to start my own db module for my current little project when I realized that there was already a nice base module available. Played around a little with it and came out with this. Not sure if it's interesting enough to merge but I thought I'd at least share it :)

In short I added `max_idle_conns` and `max_open_conns` options and a ping test of the connection on init. I also moved the rollback to use the PANIC interceptor which made it possible to remove setting `Txn` to `nil` in the `Commit` and `Rollback` since we dont need to communicate state of the txn anymore. I left a `Txn` `nil` check in the `Rollback` since a panic in `Begin` might leave `Txn` as `nil`.

I'd be happy to back and forth on this :)