shardul semwal

Results 2 issues of shardul semwal

This PR implement support for Achievements in ReteroPlayer. This PR is based on https://github.com/kodi-game/game.libretro/pull/67 , for adding support of RCheevos. This PR is used by https://github.com/garbear/xbmc/pull/126 for implementing Achievements.

This PR implement support for Achievements in ReteroPlayer. This PR is based on #67 , for adding support of RCheevos. This PR is used by [garbear-xbmc #126](https://github.com/garbear/xbmc/pull/126) for implementing Achievements.