MINOR: hlua: add simple hlua reference handling API

MINOR: hlua: add simple hlua reference handling API

We're doing this in an attempt to simplify temporary lua objects
references handling.

Adding the new hlua_unref() function to release lua object references
created using luaL_ref(, LUA_REGISTRYINDEX)
(ie: hlua_checkfunction() and hlua_checktable())

Failure to release unused object reference prevents the reference index
from being re-used and prevents the referred ressource from being garbage
collected.

Adding hlua_pushref(L, ref) to replace
lua_rawgeti(L, LUA_REGISTRYINDEX, ref)

Adding hlua_ref(L) to replace luaL_ref(L, LUA_REGISTRYINDEX)