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.