Add Support for Unity 5.3

[StanleyGoldman]

Copied from: github-beta/unity-preview#2

MethodAccessException Unity 5.3.6p3 OS X 10.11.6

Trying to open the GitHub window after import I receive the following two errors:

MethodAccessException: Method `UnityEditor.EditorGUIUtility:get_pixelsPerPoint ()' is inaccessible from method `GitHub.Unity.Utility:GetIcon (string,string)'

GitHub.Unity.Styles.get_SmallLogo () (at /Users/stanleygoldman/Documents/UnityInternal/src/UnityExtension/Assets/Editor/GitHub.Unity/Misc/Styles.cs:653)
GitHub.Unity.Window.OnEnable () (at /Users/stanleygoldman/Documents/UnityInternal/src/UnityExtension/Assets/Editor/GitHub.Unity/UI/Window.cs:80)
UnityEditorInternal.InternalEditorUtility:LoadSerializedFileAndForget(String)
UnityEditor.EditorApplication:Internal_CallGlobalEventHandler()

MethodAccessException: Method `UnityEditor.EditorGUIUtility:get_pixelsPerPoint ()' is inaccessible from method `GitHub.Unity.Utility:GetIcon (string,string)'

GitHub.Unity.Styles.get_RepoIcon () (at /Users/stanleygoldman/Documents/UnityInternal/src/UnityExtension/Assets/Editor/GitHub.Unity/Misc/Styles.cs:746)
GitHub.Unity.Window.DoNotInitializedGUI () (at /Users/stanleygoldman/Documents/UnityInternal/src/UnityExtension/Assets/Editor/GitHub.Unity/UI/Window.cs:219)
GitHub.Unity.Window.OnGUI () (at /Users/stanleygoldman/Documents/UnityInternal/src/UnityExtension/Assets/Editor/GitHub.Unity/UI/Window.cs:131)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)