Fixed invalid clients in sdkhooks on reload
Added natives to access rpg ranking

SMRPG_GetClientRank, SMRPG_GetRankCount and SMRPG_GetTop10Players enable
other plugins to access the ranking information
Added smrpg_resetstats_top10_maxlevel to resetstats

When the top 10 players total levels add together to this maxlevel, the
server is reset.