Merge pull request #64 from martiert/pullRequest

Make a CHECK_THROWS macro, which tests that we throw an exception.
Remove unreferenced variable e in the CHECK_THROWS macro.
Change to use the clang __has_attribute to check for noreturn.

The preferred way to check if clang supports noreturn is to use
__has_attribute(noreturn). Since that doesn't exist except in clang,
we need to define it to 0 for all compilers that don't have it
predefined.