[ADD/WIP] Matrix<->XMPP DMs.

There is still this fun issue with the blocking XML parser leaking
memory on pthread_cancel, I'll try to deal with that later, maybe with a
test stanza.

I also apologise for swearing in the last commit. I promise it won't
happen again, Prosody.

src/XML/SAX.c

@@ -658,8 +658,6 @@ XMLParseAttQuote(XMLexer *lexer)
 
     point = XMLInitialiseBuffer(lexer);
 
-    /* TODO: My Prosody is actually trolling me.
-     * GIVE ME A FUCKING STREAM ID YOU RETARD. */
     while ((c = XMLGetc(lexer)))
     {
         if (!IsNormalQ(c))