Preparation for bidirectional statusnet sync, functions moved from addons into the core and some converting stuff

This pull request contains many small changes:
- There are new constants for the bidirectional statusnet sync (pull request for that will follow in several minutes)
- The function "original_url" was present in several addons. It is now moved into the core
- The "parse url" functionality now checks for empty values and the values from the oembed function are taken at last (since they seem to be more helpful)
- oembed can now use an API key from embedly
- The functionality to add twitter contacts without the twitter addon is removed, since this isn't working anymore.
- "mailto"-links are now converted into [mail] elements
- HTML paragraphs are now treated correctly when converting from Diaspora