Support lua 5.2

[beatgammit]

I don't think support for lua 5.1 should be dropped, so I think something needs to be decided in how multiple lua versions will work together within rust-lua. Here are a few ideas:

1. separate branch for each lua version
   • denoted by different version scheme in cargo repository
2. separate crate per supported lua version (use lua::lua51;, use lua::lua52;)
3. separate repository for each lua version
   • change crate name to lua51 so a future lua52 crate can be implemented
4. build flags?

Of these, I'd like either 1 or 2, but if you're unwilling to support multiple lua versions, perhaps 3 would be best.

[lilyball]

I haven't looked into how different Lua 5.2 is, but if it's not too much trouble, using a feature flag sounds like the best idea to me.=