Updated how mappers for persistence work with the MapperFor attribute and have made BaseMapper public but have

kept all properties, etc.. internal just so that they can be found so we don't have to register them.
Updates unit test to ensure the plugin manager is init'd properly with the 'false' override. Ensures that the base
db test also initializes the plugin manager properly.

src/Umbraco.Tests/CodeFirst/CodeFirstTests.cs

@@ -26,7 +26,7 @@ namespace Umbraco.Tests.CodeFirst
             UmbracoSettings.SettingsFilePath = IOHelper.MapPath(SystemDirectories.Config + Path.DirectorySeparatorChar, false);
 
             //this ensures its reset
-            PluginManager.Current = new PluginManager();
+            PluginManager.Current = new PluginManager(false);
 
             //for testing, we'll specify which assemblies are scanned for the PluginTypeResolver
             PluginManager.Current.AssembliesToScan = new[]