Add DBURI role.

Implements a `db_uri` attribute that stores a URI::db object. URIs passed to
the constructor will have the various parts from the App::Sqitch `db_*`
attributes merged in to them. The default URI consists solely of those values.
Add port, remove unused class.
Install URI::db from GitHub.

Should remove this line once URI::db is released on CPAN.
Skip URI::db for now.
Add `db_uri` attribute to Engine.

And have the `destination` reference it.
Make db_uri read/write.

But don't merge it with the default when setting the value.
Still merge when the value is set via the constructor, though. At
least for now.
Switch to db_uri in the SQLite engine.
Replace db_name with db_uri in SQLite engine.

One test failing, but I think I am going to reverse some of this stuff, and
move the URI into the base engine class.