Main.cpp set up

Added main.cpp -- nothing more than a "Hello World" meant to demonstrate
that exes can be built and everything is linked properly.

Will work on getting an SFML demo working.
Mac support attempted

I'm not sure if it builds properly with Xcode now, but premake4 has all
of the links (probably) working right and can generate a project file for
it. I also decided to include the precompiled SFML static libraries (on
windows) because apparently you're not supposed to compile them yourself
without using their CMake build system. It's not illegal or anything, it's
just really time consuming and probably won't get you a good result. What
can I say, c++ libraries are finnicky.