Updated UML Diagrams for Structural Patterns

Based off @mac2000 comment in issue #47, I re-worked the UML diagrams. This time I used PHPStorm to generate them for me (I'm new to UML, so I figured I got some of them wrong). They look a little nicer too.

@domnikl If you want me to continue adding more, please let me know. I'm happy to do it.