Debugged implementation of Diaspora relayable_retractions

Debugged implementation of Diaspora relayable_retractions

Diaspora "relayable_retraction" is now supported by Friendica.
The following should now work:

	Friendica top-level post:
	Diaspora comment deleted, disappears in Friendica
	Friendica comment deleted, disappears in Diaspora
	Diaspora like retracted, disappears in Friendica
	Friendica like retracted, disappears in Diaspora

	Diaspora top-level post:
	Same

There are still exceptions, however. First, Friendica and Diaspora
seem to frequently reject comments with an "invalid signature" error.
This can probably be fixed.

Also, some comments/likes/retractions seem to just disappear on the
Diaspora side. In the Diaspora log these seem to be accompanied by a
"not a valid object" error, often preceeded by a "received a comment
but no corresponding post" error. These seem to be purely internal,
since sometimes it works for some Diaspora contacts but not others.