Fix up dub package

When I tried to use DAllegro as a dub package a few things went wrong:
- Undefined references to modules
-- fixed by adding `importPaths`
- Linkage errors to allegro functions
-- fixed by adding `libs` to reference all the C libraries (pragma(lib)s seemed to not work with dub?)

I didn't catch these before when I was just using a local dub package.
Also, I believe `dub.json` is now preferred over `package.json`