Implement multi-account support

[akrennmair]

Currently, only logging in as one specific account is possible. This should be extended so that you can login as two or more accounts and read all of their tweets. The following things should be considered:

• find a way to save the OAuth tokens for all the different accounts (one file per account, maybe?).
• provide the ability to add (and delete!) configured accounts.
• it must be possible to select as which account a tweet should be posted.
• if several configured accounts follow the same user, display tweets of this user only once.
• find a way to continuously load updates without making the UI slow (background thread? Twitter streaming API? [much development effort])