Fixing all /W4 level warnings.

This can reduce worst case peak memory usage by 30% depending on the
realloc implementation.

This should improve performance and improve readability.

Fix copy pasting with MSVC

This should fix copy pasting of the C and header files with the MSVC
compiler.

Locale independence

This first prints the number into a temporary buffer and then copies the
number to the output.

A positive side effect is that cJSON no longer reserves more space for
the number in the output than is necessary.

Because sizeof('\0') is actually sizeof(int) not sizeof(char).

Found by coverity