Code refactoring and misc changes

User model:

 The id field has been renamed to user_id.
 The update() method now supports variable no of arguments.
 json() method has been turned to a property.

User view:

 All endpoints now have a trailing slash.
 The /users and /users/user_id endpoints are routed to a single
 method which then handles them separately.

Tests:

 The update test now uses variable arguments and resets the changes.
 The db can now be deleted via the code itself.