Fixed exception in AddRange in ConcurrentObservableDictionary
This is a fix for issue #16
- Fixed ConcurrentObservableDictionary, so passing an IEnumerable<,> to AddRange doesn't cause an exception
- Went through entire code base, trimming down the number of static casts used
- Moved units tests out to their own project, as opposed to being part of the manual test app
- Added .NET Core target framework to the unit test project so as all compilation targets will be tested