Support for database URLs

With a bunch of tests.

Of note:

1. in the case of information present in both URL and "normal" parameters, I'm currently giving priority to the information from the URL; IMO this makes more sense than vice versa (base info would be defined in params, and each developer or environment has a URL that may or may not override that base info, such as charset)
1. the syntax for SQLite is `sqlite:///relativepath.db` or `sqlite://ignoredhost/relativepath.db` for relative, and `sqlite:////tmp/absolutepath.db` or `sqlite://ignoredhost//tmp/absolutepath.db` for absolute paths to the database file (I went back and forth on this, but this way is easier and more consistent; https://github.com/kennethreitz/dj-database-url does the same)
1. extra query params are simply "copied" into `$params` verbatim; makes sense IMO especially considering point number 1
1. the URL/driver mappings may need some review
doesn't need to be prefixed with `_` anymore (was an old convention)