Moved all ldap configuration to GUI

Resolved issue #18.
Moved ldapuser and ldappassword configuration in GUI. The variables for
default roles of student and employee will be moved once task #4 is
completed.
Merge pull request #36 from TriptiSharma/issue-18

Issue 18 :  Changed approved

Testing for this request included:

    1. Verifying fields appear under Network > Network Settings> CAS Client
    2. Verifying access for a new user
    3. Verifying access for an existing user with invalid LDAP values on wordpress settings
    4. Verifying security for password encryption. For this test, the new issue #39 was added.
Fix #40 - LDAP user/pass not saving

LDAP username & password settings did not seem to be saving to
the database. Added those two fields to the retrieval of
settings (they were missing), but didn't find the save code
before it started working. Jonathan set me up with dev access
to the database at that time, so I'm thinking that something
happened with the DB permissions.