Default config/database.yml fetched from the pwd

Hi!

I'm using this gem for a [personal project](https://github.com/squareteam/yodatra) but I needed to load this gem not in the usual context of a Sinatra app but in a middleware for Sinatra.

For a default configuration of  the DB file, I think it would be more generic to give a abs path (rather than a rel path) with the process working directory.

What do you think abt it?

Thanks!