Add uv_constants.h that I forgot to push

src/uv_constants.h

@@ -0,0 +1,7 @@
+#include "uv.h"
+#define XX(uc,lc) :UV_##uc,
+const UV_UNKNOWN_HANDLE = 0
+handles = [UV_HANDLE_TYPE_MAP(XX) :UV_FILE, :UV_HANDLE_TYPE_MAX, :UV_RAW_FD, :UV_RAW_HANDLE]
+for i=1:(length(handles))
+@eval const $(handles[i]) = $i
+end