added LICENSE
put short descriptions into headers, new functions

  + ArcUnit extracting functions now take struct instead of const
    pointers;
  + fixed aie_arcformats[] array declaration;
  + renamed aie_Errno member of aie_Error from 'e' to 'no';
  + aie_error() now accepts const char* datum;
  + declared generic xoring archive extracting functions;
  + added aie_ensure_dir() function, works like 'mkdir -p';
  + debugging error hook now able to print out errno when needed;
merged aie_archive-kinds.h into aie_archive-formats.h