Revert "Eliminating Concept"
Merge pull request #281 from leapmotion/revert-274-dep-concept

Revert "Eliminating Concept"
Correcting platform incompatibilities

- OnStop should be invoked after Start has been called
- Adding a void OnStop override for users inheriting CoreThread
- Sealing OnStop(bool) to prevent odd behavior in CoreThread
- Adding a protected GetOutstanding method to CoreRunnable rather than requiring direct access to its members; fixing the resulting naming collision in AutoPacketFactory