Added new "f" flag to the tar unpack, because without it doesn't complete, an compile fails

- more aggressive cache for files under CMAKE_BINARY_CACHE
  * don't re-read files MD5
  * don't repack archives
- add support for ENV{OPENCV_DOWNLOAD_PATH}
- added ID parameter as project/group identifier
- non-flat .cache directory (based on ID)
- download message prefix based on ID
- more detailed logging via ocv_download_log() macro
- force .gitignore file for .cache folder (with '*' pattern)