Implemented UUID generation (RFC4122)

This patch adds new API:
  * xmpp_uuid_gen()

examples/uuid.c

@@ -0,0 +1,23 @@
+#include <stdio.h>
+#include <strophe.h>
+
+int main(int argc, char **argv)
+{
+    xmpp_ctx_t *ctx;
+    char *uuid;
+    int rc = 0;
+
+    ctx = xmpp_ctx_new(NULL, NULL);
+
+    uuid = xmpp_uuid_gen(ctx);
+    if (uuid != NULL) {
+        printf("%s\n", uuid);
+        xmpp_free(ctx, uuid);
+    } else {
+        fprintf(stderr, "Couldn't allocate memory.\n");
+        rc = 1;
+    }
+
+    xmpp_ctx_free(ctx);
+    return rc;
+}