Provide a script for batch creating user accounts

Summary: Make it a little easier to create a bunch of accounts if your company
has more than like 5 employees.

Test Plan: Ran "add_user.php" to create new users. Created new users from the
web console.

Reviewers: btrahan, jungejason, rguerin

Reviewed By: btrahan

CC: aran, btrahan, rguerin

Differential Revision: https://secure.phabricator.com/D1336