upgraded lua-resty-redis to 0.26. 

bumped version to 0.25. 

bumped version to 0.24. 

optimize: we now alway call tostring() upon args in redis query methods. also reduced Lua string concatenations in redis query composition. Signed-off-by: Yichun Zhang (agentzh) <agentzh@gmail.com>

fixed Makefile to allow relative paths in LUA_LIB_DIR when DESTDIR is… … not specified.

doc: updated the copyright years. 

bumped version to 0.20. 

bumped version to 0.19. 

bumped version to 0.18. 

doc: markdown formatting fixes.