Next will be to get all storages (roster, history, vcards, ...) from these MemoryStorages.
And then we can replace these MemoryStorages with real FileStorages, so you can see
everything even if you're offline.

I added a new include for the PlatformCryptoProvider; we should clean up all includes and
only use specific files instead of just everything from Swiften: this will speed up
compilation much!