[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/HttParsee.c

@@ -61,7 +61,7 @@ ParseeRequest(HttpServerContext *ctx, void *argp)
 }
 
 HttpClientContext *
-ParseeCreateRequest(ParseeConfig *conf, HttpRequestMethod meth, char *path)
+ParseeCreateRequest(const ParseeConfig *conf, HttpRequestMethod meth, char *path)
 {
     HttpClientContext *ctx;
     if (!conf || !path)