Android: Added sf::getNativeActivity() to retrieve a handle to the current activity

This is only implemented for Android builds and requires the user to include the sub-header specifically. Other platforms will trigger a compiler error, if they try to include this header file.

examples/android/AndroidManifest.xml

@@ -10,6 +10,8 @@
     <uses-sdk android:minSdkVersion="9"
               android:targetSdkVersion="19" />
 
+    <uses-permission android:name="android.permission.VIBRATE" />
+    
     <application android:label="@string/app_name"
                  android:icon="@drawable/sfml_logo"
                  android:hasCode="false"