Make assertions a little more expressive
Change Activity API: registeredMembers is now a function, not a direct property access
Extract registeredMembers into Resource object (rather mechanically for the first step)
Fix loading of registeredMembers from Resource
Move member add/remove from Activity to Resource
Use dictionary to store Resources instead of array (allows acces by name)
Make modes of Activity construction explicit
Change values of exisiting activity object instead of recreating it

Fixes bug that we lose registered memebers when editing the activity.
Note areas for improvement
Merge pull request #448 from wosc/limited-activities

Introduce Resources for Activities