Firing off a thread instead of hashing while blocking

renamed sink download_hash to hash_download to avoid confusion
move hashing section and requires of core/async.lua to a core/hash_thread.lua
adding vapor.currently_hashing table along with handlers
adding hashing icons (4x)
adding hasing status state
stopping system from execgame when hashing
moving main.lua:round to core/hash_thread.lua as that's the only place it's used.
  Consider making a shared library for this?
adding thread hashing handlers (hasher)
moved src/lib/hash.lua.[info|patch] to dev/hash.lua.[info|patch] to avoid
  physfs bug (naming issues). Thanks @Boolsheet and @slime73
  This info / patch shouldn't be in here anyway, imo.
Changed MB to MiB (Thanks @Boolsheet)

Closes #41
Allows windows to ALT-F4 (and close internally for other OS's)
Closes #121