[ASNETutorial]Application always crashing after changing orientation (rotating device)

Hello again.

How to reproduce bug:
Open example project (https://github.com/gorbin/ASNETutorial) on device, rotate device at least once and then press any of the social buttons.

Result for facebook:

```
Process: com.github.gorbin.asnetutorial, PID: 13770
java.lang.NullPointerException
at com.facebook.Session$AuthorizationRequest$1.startActivityForResult(Session.java:1902)
at com.facebook.Session.tryLoginActivity(Session.java:1409)
at com.facebook.Session.authorize(Session.java:1159)
at com.facebook.Session.open(Session.java:1237)
at com.facebook.Session.openForRead(Session.java:471)
at com.github.gorbin.asne.facebook.FacebookSocialNetwork.requestLogin(FacebookSocialNetwork.java:149)
at com.github.gorbin.asne.core.SocialNetwork.requestLogin(SocialNetwork.java:205)
at com.github.gorbin.asnetutorial.MainFragment$1.onClick(MainFragment.java:160)
at android.view.View.performClick(View.java:4658)
at android.view.View$PerformClick.run(View.java:19461)
at android.os.Handler.handleCallback(Handler.java:733)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:146)
at android.app.ActivityThread.main(ActivityThread.java:5653)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
at dalvik.system.NativeStart.main(Native Method)
```

For VKontakte will be something like

```
Caused by: java.lang.NullPointerException: Argument 'currentActivity' cannot be null
at com.facebook.internal.Validate.notNull(Validate.java:29)
at com.facebook.Session.onActivityResult(Session.java:723)
at com.github.gorbin.asne.facebook.FacebookSocialNetwork.onActivityResult(FacebookSocialNetwork.java:535)
at com.github.gorbin.asne.core.SocialNetworkManager.onActivityResult(SocialNetworkManager.java:92)
at ru.**MyPackage***.MyLoginActivity.onActivityResult(LoginActivity.java:75)
at android.app.Activity.dispatchActivityResult(Activity.java:5734)
at android.app.ActivityThread.deliverResults(ActivityThread.java:3696)
            at android.app.ActivityThread.handleSendResult(ActivityThread.java:3743)
            at android.app.ActivityThread.access$1400(ActivityThread.java:172)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1356)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:146)
            at android.app.ActivityThread.main(ActivityThread.java:5653)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
            at dalvik.system.NativeStart.main(Native Method)
```

after finishing fragment.