Added placeholder models for Admin and Clerk

@codesliced Up for review. Will get a Beatles song for the next one. :-)

Also, a few fixes:

- `Gemfile.lock` updates (run `bundle install` again)
- Fixed the double `CreateStudent` migrations
- Changed `birth_date` field in `Student` to match `birthday` in the seeds

Requires the following:

`rake db:drop; rake db:create; rake db:migrate; rake db:seed`