Make core Ticker abstract and public (#116)

* Make core Ticker abstract and public
Make the core Ticker abstract and public so it can be inherited by platform
implementations; remove now-unused parts of original Ticker; add CreateTicker
to IPlatformServices; remove unused CreateTimer methods from IPlatformServices

* Add docs for Ticker

* Remove unnecessary Ticker.Default set

* Move Ticker into Internals

* Update Ticker docs

* Remove old Ticker docs

* Remove commented code

1 files changed, 5 insertions, 4 deletions

diff --git a/Xamarin.Forms.Core.UnitTests/MotionTests.cs b/Xamarin.Forms.Core.UnitTests/MotionTests.cs
index acb764d7..0d8d476c 100644
--- a/Xamarin.Forms.Core.UnitTests/MotionTests.cs
+++ b/Xamarin.Forms.Core.UnitTests/MotionTests.cs
@@ -1,25 +1,26 @@
 using System;
 using System.Collections.Generic;
 using NUnit.Framework;
+using Xamarin.Forms.Internals;
 
 namespace Xamarin.Forms.Core.UnitTests
 {
 	internal class BlockingTicker : Ticker
 	{
-		bool enabled;
+		bool _enabled;
 
 		protected override void EnableTimer ()
 		{
-			enabled = true;
+			_enabled = true;
 
-			while (enabled) {
+			while (_enabled) {
 				SendSignals (16);
 			}
 		}
 
 		protected override void DisableTimer ()
 		{
-			enabled = false;
+			_enabled = false;
 		}
 	}