@kloepper pointed out a while back that every compiler you've ever heard of supports this. Plus, it's more concise, it's not prone to copy-paste errors, and it looks nicer.

At the time I wanted to remain consistent and I didn't feel like spending the time to update all my existing code. But, every time I've added a new header since I've cursed the include guard, so I finally broke down and changed it.

This prevents the compiler from reporting warnings in these headers while compiling application code. Hopefully this will stem the never-ending stream of complaints from people who enable pedantic warnings.

Response to comments on #144. Remove 'using namespace std' from header, added additional tests, updated comment style, and added self to CONTRIBUTORS.