Support Lua allocator

[agladysh]

Currently lua-cmsgpack uses malloc. This is not very nice to users who configure Lua state to use their own allocator.

Use lua_getallocf instead: http://www.lua.org/manual/5.1/manual.html#lua_getallocf

Here is an example: https://github.com/agladysh/luabins/blob/master/src/savebuffer.c#L96

[fperrad]

see http://github.com/antirez/lua-cmsgpack/pull/20 where I remove the need of allocation