Work on #226, #213, added references of Gang Of Four to all GoF patterns, added tag for Gang Of Four, added difficulty tags to some of them

- properly categorize all patterns
- remove pattern list from readme
- minor fixes to readme
- removed "introduction" because its not a pattern and an error i
committed some time ago

This makes cleaner URLs thanks to jekyll

I did the job and splitted up the readme, hopefully everything was
splitted correctly...