Fix some functions when using cgreen with its namespace in C++.

[TommyJC]

Hello,

Using cgreen for C++ without including using namespace cgreen in the code results in compilation errors when one attempts to use cgreen::assert_that().

I just prefixed some things with cgreen:: and added namespace cgreen { where appropriate.

The changes from this patch fixed the parts of cgreen I utilized but I haven't been able to test everything in cgreen.

Ideally I wanted to add the cgreen namespace to TestSuite in include/cgreen/internal/suite_internal.h as well but since that has namespace cgreen { later, I take it there was a good reason not to add TestSuite to the cgreen namespace.

Those who are applying using namespace cgreen to get around the compilation error should see no difference.