Feature parity

[natecraddock]

How much should we cover up the differences between Lua versions?

lua_len isn't available in Lua 5.1. Should Lua.len() support this by calling rawLen or objlen?

[robbielyman]

ah, I stepped in this too with lua_equal in #118. I think there's a valid case to be made either way... I think maybe it's correct to push the complexity introduced by inconsistencies between versions onto users—if they want to support compiling against different Lua versions, they should be aware that they're opening themselves up to real differences in behavior?