Issue #4 CSV Enrolment Import

I have implemented CSV Enrolment Import where instructors and
admins can import existing or new users into a course. Some checking
has also been done such as whether the user already exists or whether
the user is already enrolled. After the user press upload and the
file has been processd, the page will update to render a result
page that displays all the users that were successfully enrolled,
and the users that failled to be created or enrolled with their
accompanying error messages.

Some details include the following:
- user already exist, no attributes change - marked as successful
- user already enrolled, not re-enrolled - marked as successfull
- display name conflict, append a random number from 1000 to 9999
- passwords are auto-generated
- display names defalt to first name + last name

The page can be accessed from the course home page.
Modified create_user and enroll_student to use "import methods"

I have modified create_user and enroll_student to use the import
methods import_users and enrol_users. This is so so that users
will be using the same methods whether they are creating users
and enrolling students via the interface or via import. Due to this
change, the controller also has been slightly modified to accept
different return values for successes and errors.