c++11 fixes on gcc-491, glibc-2.12-1.132.el6

Hi Larry, 

Just a few fixes I had to make in order to compile under -std=c++11 on GCC 4.9.1 - Centos 6.5.

The using declarations inside of `missing_math.h` were particularly nasty, as they leak out into the public interface. I don't have any way to test this on Windows, but the declarations I removed seemed to already exist in the OIIO namespace, so maybe we should just patch the places where it fails to compile with the relevant OIIO namespace.

I've already changed some of the calls in idiff and oiiotool to use OIIO::isnan / OIIO::isinf respectively. 

Cheers,
Mark
I'm not at all certain, but they're forcing any users of the public API to have their current namespace trampled by the contents of those using declarations.
`imagebuf.h` includes `fmath.h` which includes `missing_math.h`.
If they are required, we should put them in the OIIO namespace, and change any code that calls it.