enhanced support for C++11 style nullptr

with these changes you can now write the following tests:

std::exception_ptr ex;
CATCH_REQUIRE(ex == nullptr);

and

std::unique_ptr<int> ptr;
CATCH_REQUIRE(ptr.get() == nullptr);

The issue was that you cannot cast a std::nullptr_t to a non-const
reference.

I am unsure, if I need to commit all 4 files (including changes to README, version and single header file) or just the file that contains the actual fix...