Let's try rake-compiler

The mingw loonies have won :( Time to kick MSVC out of the gem build process.

So let's see if I can refactor Gosu to use `rake-compiler` for the source and Windows gems.

- [x] Move files around
- [x] Fix `.podspec` for OS X testing
- [ ] Get `rake-compiler` to work on OS X
- [ ] Get `rake-compiler` to work on Linux
- [ ] Make C++ Gosu work again on Windows with MSVC (for C++ dev)
- [ ] Make C++ Gosu work `mingw` on Windows (GDI+ trouble?)
- [ ] Try cross-compiling Windows gem

If this fails, I should keep the modern directory layout regardless.

(Note: If I want to ship fat, pre-compiled Mac gems - and I do! - that will be another project.)