Added AbstractCRDT base class with ClientID generator
Adding NuGet package config
Clarifying GCounter equality.

After conversation with @seancribbs and @OJ I've simplified the
GCounter.Equals to use a combination of Dictionary.Count, key intersection
count, and then strict dictionary equality, as opposed to the partial
order equality stated in Shapiro, et al.
GCounter tests & Merge clean up

* Added additional GCounter tests
* Modified GCounter.Merge to accept an optional clientId parameter.
Renamed GCounter.cs to GCounterTest.cs
Clean up

* Cleaned up String.IsNullOrEmpty checks
* passed clientId to GCounters during PNCounter merge