Project updates

I made these updates for my own maintenance, but they might come in handy.

- Organizes the code into `include/` and `src/`
- Adds a Makefile (only tested on OS X) that will build `libartemis.a` to the `lib` directory
- Adds `include/Artemis.h` that has all header files included

To properly use Artemis.h you'll want to copy the header files to something like `/usr/local/include/Artemis/`. I'm hoping to add `install` to the Makefile at some point to automatically copy the headers and lib to the proper paths.