Fix crash on App startup when no external storage is available

This fixes Issue #40 and **only** Issue #40.

The MainMenuActivity is now hardened against unmounted storage, the rest of the App is **not**.

No tests were added because I couldn't find a way to fake the return value of externalStorageAvailable(). I tried [PowerMock](https://code.google.com/p/powermock/) + [Mockito](https://code.google.com/p/mockito/) + [Dexmaker](https://code.google.com/p/dexmaker/) but couldn't get it to build.
Like the commit message says, I decided to fix this issue right now because it's critical. With no way to test, it makes little sense to harden the other activities at this point in time.

A new Issue will be opened for the rest of the app.

[Successful testrun](http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/98/)