Use rand() for randombytes

[danimtb]

I dont know if rand() is multiplatform, but I had problems with defines for WIN32...
http://www.cplusplus.com/reference/cstdlib/rand/

So I guess that better to keep it simple. WDYT?

Yes, I took this example from other repo using this library. But I would do this:

diff --git a/test_package/example.cpp b/test_package/example.cpp
index af944d8..b7490d3 100644
--- a/test_package/example.cpp
+++ b/test_package/example.cpp
@@ -4,21 +4,24 @@ extern "C" {
 #include "tweetnacl.h"

 /* Library needs external randombytes implemented */
-#include <stdio.h>
 #include <stdlib.h>
 #include <time.h>

 void randombytes(unsigned char * ptr,unsigned int length)
 {
+    int i;
     srand(time(NULL));
-    ptr = new unsigned char(rand());
+
+    for (i = 0; i < length; i++) {
+        ptr[i] = (unsigned char)rand();
+    }
 }

 #ifdef __cplusplus
 }
 #endif

-
+#include <stdio.h>
 #include <stdexcept>
 #include <string.h>

Are you OK if I change this PR?

[danimtb]

test_package was failing with:

Generating random data failed. Please report this to https://github.com/ultramancool

[danimtb]

good for me!