ADDED: force update checks file integrity

If force update is enabled:
 * AssetDownloader() compares existing assets' size and hash to
   remotely reported size and hash. Comparing to locally  saved hash
   also work.
 * gatherAssets() compares assest hashes to locally saved hashes if
   possible. Otherwise asset is added into list and checked by
   AssetDownloader().
Merge pull request #722 from jikuja/fix_check_assest_with_force_update

ADDED: force update checks file integrity