v2 refactoring

[szuecs]

The current lib is simple and works for a "golden" path like stack, but does not work for:

• non Gin handler applications (Other web frameworks #31 and net/http middleware functions #47)
• not able to pass a logger can I replace the log package with zap? #75
• we are a bit stuck by using globals instead of having a struct

The best would be to not break the API but have an additional API that can support all these cases and we use the old way to create the old defaults.