Implement LuaPlainString for non-UTF-8

[mkpankov]

Fix #135

[tomaka]

As discussed in #135 I don't think Vec<u8> should represent a String.

You should create a newtype like pub struct LuaNonUtf8String(Vec<u8>); for example, and use it instead.

EDIT: LuaNonUtf8String would suggest that it can't contain an UTF8 String, so more LuaAnyString or something instead I guess.

[tomaka]

Thanks