Add LDAP authentication

Added the devise_ldap_authentication gem and configured according to
the instructions at
http://wiki.phys.ethz.ch/readme/devise_with_ldap_for_authentication_in_r
ails_3
Connected the user model with the employee model and verified that LDAP
authentication will in fact work.
Completed login and admin logic

What's new:
- Authentication logic is basically complete
- Login flow requires user to update their profile the first time they
log in
- Added admin field to user table to allow addt'l privs
- Only admins can edit all profiles/fields, otherwise users can only
edit their own
Merge branch 'ldap_authentication'