Add signed flags to MSVC integer definitions

[ibell]

This is important because the integer definitions clash with the integer definitions in the rapidjson C++ library, but only at the level of signed/unsigned integers. We use both libraries in the CoolProp library

[redboltz]

@ibell, thank you for sending the PR. I will merge it.

Before merging, I'd like to confirm a point. I think that the actual problem is only happened __int8. Because it is a typedef of char, and char, singed char, and unsigned char are different types. See https://msdn.microsoft.com/en-US/library/s3f49ktz(v=vs.90).aspx
However, other types without signed keyword such as __int16 are equivalent to signed types, in this case it is a typedef of signed short int.

The PR added all types to singed for a uniformity.

Am I understanding correctly?

[ibell]

I know that the problems happen for me with int8_t, and I figured it made sense to fix them all. And yes, it seems pretty much only for VS9 where I have problems. Other versions of MSVC work fine.

[redboltz]

Thank you for the clarification :) merged.