A hash is stored for each native library with the name libraryName.hash.
If the library is not found on the system search path, it is extracted to a cache directory.
Extracted libraries are named with the hash appended, so the library will not be
re-extracted if one with the same hash already exists.
Hashing without the hash file requires double traversing if the file is not in the cache,
but it is still faster than creating a new file in most cases. This won't be needed
after opencv is updated to provide a hash as well.
