allow providing assertion detail using operator <<

Adds support for operator << to provide assertion message instead of the current extra string parameter. This can be done by adding support for operator << to CppUT::Impl::CheckerFileLocation. As operator << as a lower priority than + and / it will be evaluated after the other part the assertion expression.

Basically, we want to be able to write:
CPPUT_ASSERT( x % 2 == 0 ) << "Value should be even: " << x;