Implemented UUID generation (RFC4122)

This patch adds new API:
  * xmpp_uuid_gen()

src/uuid.c

@@ -0,0 +1,70 @@
+/* uuid.c
+ * strophe XMPP client library -- UUID generation
+ *
+ * Copyright (C) 2015 Dmitry Podgorny <pasis.ua@gmail.com>
+ *
+ *  This software is provided AS-IS with no warranty, either express
+ *  or implied.
+ *
+ *  This program is dual licensed under the MIT and GPLv3 licenses.
+ */
+
+/** @file
+ *  Generation of UUID version 4 according to RFC4122.
+ */
+
+#include "strophe.h"
+#include "common.h"
+#include "rand.h"
+
+/** @def XMPP_UUID_LEN
+ *  UUID length in string representation excluding '\0'.
+ */
+#define XMPP_UUID_LEN 36
+
+/** Generate UUID version 4 in pre-allocated buffer.
+ *
+ *  @param uuid pre-allocated buffer of size (XMPP_UUID_LEN + 1)
+ */
+static void crypto_uuid_gen(xmpp_ctx_t *ctx, char *uuid)
+{
+    uint8_t buf[16];
+    int i = 0; /* uuid iterator */
+    int j = 0; /* buf iterator */
+
+    static const char hex[] = {'0', '1', '2', '3', '4', '5', '6', '7',
+                               '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
+
+    xmpp_rand_bytes(ctx, buf, sizeof(buf));
+    buf[8] &= 0x3f;
+    buf[8] |= 0x80;
+    buf[6] &= 0x0f;
+    buf[6] |= 0x40;
+    while (i < XMPP_UUID_LEN) {
+        if (i == 8 || i == 13 || i == 18 || i == 23)
+            uuid[i++] = '-';
+        else {
+            uuid[i++] = hex[buf[j] >> 4];
+            uuid[i++] = hex[buf[j] & 0x0f];
+            ++j;
+        }
+    }
+    uuid[XMPP_UUID_LEN] = '\0';
+}
+
+/** Generate UUID version 4.
+ *  This function allocates memory for the resulting string and must be freed
+ *  with xmpp_free().
+ *
+ *  @return ASCIIZ string
+ */
+char *xmpp_uuid_gen(xmpp_ctx_t *ctx)
+{
+    char *uuid;
+
+    uuid = xmpp_alloc(ctx, XMPP_UUID_LEN + 1);
+    if (uuid != NULL) {
+        crypto_uuid_gen(ctx, uuid);
+    }
+    return uuid;
+}