Calling `Conifgure` twice raises an exception

[omerts]

We have an ioc container, and when it tries to initialize our VWO adapter for another instance we get:

System.ArgumentException: An item with the same key has already been added. Key: cl
at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
at VWOSdk.VWO.SetUsageStats(String key)
at VWOSdk.VWO.Configure(ILogWriter logger)

As a workaround we have made it singelton, but this shouldn't crash, or at least give a way to figure out that configure has already been called.

[omerts]

I saw that this has already been fixed, but not yet released to nuget. Can you please add to nuget?
Also, if you want i made the function a little better, not creating a new dictionary in case key already exists:
#7