[GEARPUMP-299] Update gearpump.apache.org page for 0.8.3 release

- (c) header updated to 2017
- version changed to 0.8.3
- regenerated content
- changed link to release notes
- changed download.md

Author: karol brejna <karol.brejna@intel.com>

Closes #13 from karol-brejna-i/GEARPUMP-299.