created seed file with users, repos, versions; fixed models

* Version model now has <code>attr_accessible</code> for <code>:user_id</code>, <code>:repository_id</code>
* Repository model now has  <code>attr_accessible</code> for <code>:creator_id</code>
* Seed file makes 5 users, 2 repos, 4 versions randomly assigned to users/repos