Add #getUsersLookup() and fix #getUsers()

Get users was previously throwing an error when you omitted either `user_id` OR `screen_name` (not AND), and I added support for the [`users/lookup`](https://dev.twitter.com/docs/api/1.1/get/users/show) endpoint. Also, `#getUsers()` erroneously required an `id` parameter, not `user_id` ([ref](https://dev.twitter.com/docs/api/1.1/get/users/show)).

I also laid the groundwork for some testing of this package, as well as a few tests supporting my changes.  Maybe this can be flushed out?

The `composer.json` doesn't list `phpunit` as a dependency (despite the presence of a `phpunit.xml`), so I'm not sure what versions are used elsewhere, but all tests currently pass under PHPUnit `3.7.24-4-g780cdb1` with PHP `5.3.10`.