plugin architecture

All features should be loaded via a suitable plugin architecture.
I think [setuptools.entry_points](http://peak.telecommunity.com/DevCenter/PkgResources#entry-points) with possibly [plugins](https://pypi.python.org/pypi/Plugins) is suited for the job.

components that should be in core:
- main function that can be called with a initialization path for the plugin system
- plugin architecture

features that should be accessible via the plugin architecture
- main application window as notebook
- matplotlib view-window
- matplotlib twiss view
- pymad model
- single element popup view
- mirko envelope comparison overlay
- online dll wrapper (possibly private)
- models (possibly private)