replace snprintf with a C++98 equivalent

[eelstork]

snprintf is not consistently available on Windows.
Although patchy support is possible (search snprintf in suggested reference), I find that snprintf may be advantageously replaced by a solution which doesn't involve defining arbitrarily sized buffers and converting between C and C++ strings.
As I understand that C++ streams are not very popular and the main use of snprintf in Caffe is producing numbered batches, I suggest the introduction of a utility function (format_int).

[ronghanghu]

IMHO snprintf is a standard io function in C/C++, available in <stdio.h> / <cstdio> and is (or at least should be) cross-platform. http://www.cplusplus.com/reference/cstdio/snprintf/

Note: Microsoft has finally implemented snprintf in Visual Studio 2015.

[eelstork]

@ronghanghu specifically, snprintf is available in C99 and C++11, not C++98 (for proof, replacing <stdio.h> with <cstdio> will break C.I. builds).
VS2015 is a bit too new for most Windows build workers.
This is a compatibility issue.

[eelstork]

Opinions on this PR?

[ronghanghu]

This all looks good to me. I'll try to review during the Thanksgiving (got a bit overwhelmed myself these days since it is near the end of the semester).

[eelstork]

@ronghanghu thanks! Don't forget to relax during holidays.

[ronghanghu]

Thanks @eelstork !