API: reworked

The API should now work with much more clients than before. Personally I tested with AndStatus. The return values now are mostly identical to the ones from status.net servers. Additionally I began to throw out error messages. (when a user isn't found and such stuff)

It isn't complete by now - but it is much more better than before.

**Attention**

This release adds a new table. This table contains all authors and owners of the postings - not only your contacts. Since this can get big, the table isn't tied to a user - but is valid for all users of a server. This table is especially important for networks where you can receive postings from users that you haven't added as a contact. This happens with comment postings in Friendica, Diaspora or Facebook.