Add Proguard config for Android app-compat-v7:r21 lib

[trietbui85]

[Felipe Duarte]

+1

This is a suggestion from Chris Banes:

-keep public class android.support.v7.widget.** { *; }
-keep public class android.support.v7.internal.widget.** { *; }
-keep public class android.support.v7.internal.view.menu.** { *; }

-keep public class * extends android.support.v4.view.ActionProvider {
    public <init>(android.content.Context);
}

source: https://code.google.com/p/android/issues/detail?id=78293

But I'd like to see what are you guys using for it.