Added Pin object for dealing with maps! 

I was impressed by your .Net API, it made getting started with Ducksboard quick and fun! Eventually I needed to make a map, and in the docs in says that you should use the Numbers object. I was unable to make that work because the API calls for a more complex structure than that. So I dove into your repo and created a new Pin object! 

 - I did my best to follow your conventions and modeled this on the Timeline object. 
 - I also did my best to fully implement all the options defined in the Ducksboard API.
 - I added an xunit integration test for this and verified that it passes
 - I am also using this in production. 

Please consider merging this in. Thanks!