Improve account handling

[ipoerner]

Currently, ActiveMailbox relies on the user to provide an external bridge to IMAP accounts, for example to a database containing the user data. However, there's currently no easy way to connect to an arbitrary account without creating this whole setup beforehand (as a matter of fact, the process is also not properly described yet).

The library should come with a really basic, stub-like replacement class that can be temporarily used for that matter.