Move lufa to submodule

This removes `LUFA/*`, replacing it with a git submodule of the entire LUFA tree at `vendor/lufa`. This fixes #12 and generally seems cleaner to me. It's currently a checkout of the `LUFA-140302` tag, but upgrading/downgrading is a matter of `git checkout` in that directory.

This has the effect of moving the LUFA source to `../vendor/lufa/LUFA` from the perspective of `FlySight/*`. I updated the `makefile` accordingly but didn't see any such references in the Visual Studio project files.