#195 ShouldImplement assertion incorrect
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
Replaced reader var with (*io.Reader)(nil).
Updated ShouldNotImplement.

Checks for ptr on actual and get one if needed.
Merge pull request #196 from gisakulabs/ShouldImplement_fix

#195 ShouldImplement assertion incorrect