Darwin Build

Explains/Better Enables building on the OS X Platform. I’ve written a Darwin.mk file for the purpose, given one is referenced in the Makefile but doesn’t actually exist yet. I presume this is due to Toxic’s recent move away from configure scripts?

Anyhow. It’s not perfect, and it could be prettier, but it should fix #145 going forwards. Open to criticism/thoughts/go aways/etc.

The last remaining error to compile is something that needs to be fixed by Toxic itself, which is:

```
  CC    chat_commands.o
/Users/Dominyk/Downloads/toxic/src/chat_commands.c:137:17: error: use of undeclared identifier 'TOX_GROUPCHAT_TYPE_TEXT'
    if (type == TOX_GROUPCHAT_TYPE_TEXT)
                ^
```

If we get that out of the way, with the new files in place, everything is rosy.