Move synchronisation to a service and implement auto background synchronisation

We should move all the code for online synchronisation (and probably also for SD card synchronisation) to a Android Service. This makes the ui easier (never think of hitting the sync button again) and more consistant to other Android applications.

What we gain:
* Auto background synchronisation
* Easier to set up a server for sync (with Android Accounts UI)

Links:
http://developer.android.com/resources/samples/SampleSyncAdapter/index.html
http://stackoverflow.com/questions/5504569/service-on-android
http://developer.android.com/reference/android/app/Service.html
http://developer.android.com/reference/android/accounts/AccountManager.html