Improve Plug-In architecture

[casaout]

@RaphaelRo Suggested to improve the plug-in architecture to make an independent development of trackers easier.

We are already using a plug-in-like architecture (i.e. one tracker per project that registers with a single line of code in PersonalAnalytics) and implements the ITracker interface.

@RaphaelRo Please update this issue to document your suggestions.