Start course tests

Course tests
Entry tests

Add entry tests
Fix practice and practice_response functions, also tested manually.
Merge branch 'back-end-working' of https://github.com/lexloren/E99-Language-Learning into back-end-working
Changed name of “grades.php” to “grade.php” for consistency.
Modifications for code consistency, clarity, etc.
Merge remote-tracking branch 'origin/master' into back-end-working
Implemented APIUnit.
Deleted deprecated flat APIs.
Minor edits, improvements, comments, etc.
Corrections, additions, improvements, etc.
Implemented the directory of users.
Updated for change to database (regarding “user_entry_id” column on “user_entries” table).
Enforced singletons for each grade in select_by_id.
Comments, corrections, suggestions, etc.; stylistic modifications for consistency and readability.
Updated for select() => select_by_id().
Added/modified stubs.
Modified to support addition of user_entry_id column to database.
Minor corrections, improvements, etc.
Exposed course unit APIs.