Fixed Twitter#getUsers() and added a test

`Twitter#users()` previously threw an error if you passed either a
`screen_name` or an `id`. According to the Twitter API spec, you can
pass either a `screen_name` or an `id`.

I added a test that ensures that both strategies get appropriately
passed to the #query method.
Added 'users/lookup' helper method and more tests.

First, the twitter spec requires that 'users/show' have a parameter
named `user_id`, not just `id`.

Second, added a new sugar method `#getUsersLookup` which maps to
`users/lookup`, as well as tests.

Third, added a new testcase for both `#getUsers()` and
`#getUsersLookup()` that ensures that an exception is thrown when
required parameters are not present.
Merge pull request #6 from clarkf/master

Add #getUsersLookup() and fix #getUsers()