Update cursor.lua

Add cursor:skip(n) function
Update cursor.lua

skip function fixed
Update colmt.lua

Add col:find_skip()
Update cursor.lua

fix cursor skip
use bson encode from https://github.com/tcoram/bson-lua
Merge branch 'dome'

Conflicts:
	lib/resty/mongol/colmt.lua
- fixed random crash by ll.lua's assert
- fixed cannot load by init_by_lua, because used ngx.var.pid in module file scope, so moved to function run time scope