Added AndroidFragmentType msbuild property

The upcoming code behind generation feature requires knowing the type of `Fragment` to use in some cases.  By default it uses `Android.App.Fragment`, but if the user installs the support fragment package, we want to default to use `Android.Support.V4.App.Fragment` instead.

This inclusion of a msbuild property should override the default and use the support library version instead.

Author: Redth

support-fragment/nuget/merge.targets

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <AndroidFragmentType>Android.Support.V4.App.Fragment</AndroidFragmentType>
+  </PropertyGroup>
+</Project>