Start work on Sqitch::Target.

Most of the attributes currently in App::Sqitch and App::Sqitch::Engine
will be moved here, to follow the config path options, target, engine, core,
default.
Add options attribute to App::Sqitch.
Rename t/target.t to t/target_cmd.t.
Get Target to compile.
Test Target basics, defaults.
Add `file` attribute to Plan.
Fix test.
Fully test target contstruction.

Should properly handle all combinations of target names and URIs, command-line
options, engine specs, and configuration. The --db-* options are also
deprecated, and the `uri` new methods delegate to the URI attribute, to make
the Target interface the main one one would use.
Test and fix attribute config hierarchy.

These attributes get their values from:

1. command-line option
2. target config
3. engine config
4. core config
5. reasonable default

Need to add --registry option to make it complete.
Add --registry option.
No longer a todo test.
Replace engine attributs with target attributes.

Core engine and SQLite tests now passing. The requirement that a target object
be passed to the engine means things need to be fixed in a lot of places, so
expect failures for the next few days as I work though it all.

Also:

* Change `--db-client` to `--client`.
* Make Sqitch a weak ref in Engine.
* Fix a typo in the localized duplicate project URI error message.
Get PostgreSQL engine working with Target.

Note that the deprecated username, password, host, port, and db_name keys in
the core.pg config have been removed, since Target does not support them.

Related changes:

* `registry_destination` is now a true alias for `destination`, rather than
  `uri->name`.
* Un-deprecated the `--db-*` options. I think they are just too useful and
  ingrained. I might change my mind again, though.
* Fixed Target where it was checking the wrong variable for the presence of
  a name.
Get mysql tests passing with Target.
Update vertica engine to work with Target.
Update Oracle engine for Target.
Restore option deprecations.
Remove deprecation warnings from Postgres tests.

And restore the checking of the proper name variable in Target.
Eliminate deprecation warning sin mysql test output.
Support and warn on deprecated engine configs.