Completion comparison operators

Implements `__eq__` and `__ne__` for Completion object.

It works by checking whether `full_name`, `line` and `column` are equal. I think that is a good approach to consider two objects equal-valued.

If you execute the same `complete()` command twice, the returned objects are equal when using the `==` or `!=` operators, but not when using the `is` operator.