Merge pull request #238 from hwhw/master

FFI-based MuPDF API
remove old unused code

- emulator code is now fully done via FFI. This allows us to get
  rid of some build infrastructure still in the Makefiles and of
  old input emulator code in the previously combined Lua/C API
  codebase.

- mupdfimg is now implemented in the mupdf FFI API