Methods to open individual libraries not working.

[giodamelio]

ce68dd4 added a bunch of functions to allow easily loading specific Lua libraries. They don't seem to be working right. This works fine.

extern crate hlua;

use hlua::Lua;

fn main() {
    let mut lua = Lua::new(); 
    lua.openlibs();
    let num: i32 = lua.execute("return string.len(\"aaa\")").unwrap();
    println!("{}", num);
}

If you try to load just the string library with the open_string function, it doesn't seem to load the library.

extern crate hlua;

use hlua::Lua;

fn main() {
    let mut lua = Lua::new(); 
    lua.open_string();
    let num: i32 = lua.execute("return string.len(\"aaa\")").unwrap();
    println!("{}", num);
}

This code returns the following error

   Compiling lua_test v0.1.0 (file:///home/giodamelio/tmp/lua_test)
    Finished dev [unoptimized + debuginfo] target(s) in 4.88 secs
     Running `target/debug/lua_test`
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ExecutionError("[string \"chunk\"]:1: attempt to index global \'string\' (a nil value)")', /checkout/src/libcore/result.rs:859
note: Run with `RUST_BACKTRACE=1` for a backtrace.

[giodamelio]

I've got a patch for this mostly done, see #147