feat(#24): INCR with initial expiration

[alexist]

Implementation of #24

The lua.shared semantic has evolved. The method incr now accept an optional parameter init_ttl :

syntax: newval, err, forcible? = ngx.shared.DICT:incr(key, value, init?, init_ttl?)

init_ttl allow to create the missing key with a ttl and avoid to make another call to handle expiration (incr than expire)

remaining, err = dict:incr(key, -1, limit, window)

A new parameter has been added to INCR command in order to be compliant with lua.shared.DICT