User based settings

[noplanman]

Save options per user, so multiple users can cross post posts to their own diaspora* accounts.

[noplanman]

This change is a big one!

WP to diaspora* meta data for all posts needs to be updated and assigned to a certain user. This is necessary since multiple users can share the same post to their individual accounts, which in turn requires the post to have individual meta data for each user.
This can become a bit difficult to manage on active sites with multiple users.
A clean-up feature will also become necessary, for the case that users get deleted etc. (We can probably tap in to the default behaviour when deleting a user and reassigning all their posts to another user.)

It probably makes sense to join this change together with #38 and #39.