Passes assertions package tests, updated example and fixed message.

1. Corrected the handling for ShouldNotImplement
2. Added checks to prevent actual being passed as nil
3. Added a new message and updated the current message
4. Updated examples/assertion_examples_test.go
4.1 added a comment to make asserts on interfaces clearer
4.2 changed the var pointer to nil assignment to an inline (*io.Reader)(nil) to show a simpler approach
5. Passes all tests in assertions package