Implemented chdir bindings for #170

Note: It might be best to move this binding out of `fs` and maybe into something specific for the system.

src/bindings/system/fs.rs

@@ -1,5 +1,6 @@
 use rlua::prelude::*;
 use std::sync::Arc;
+use std::env;
 use std::fs;
 use serde_json;
 use rlua_serde;
@@ -63,6 +64,10 @@ pub fn init(lua: &Lua) -> ::Result<()> {
         Ok(lua.create_string(&String::from_utf8_lossy(&data[..]).to_owned().to_string())?)
     })?)?;
 
+    module.set("chdir", lua.create_function(|_, path: String| {
+        env::set_current_dir(path).map_err(LuaError::external)
+    })?)?;
+
     module.set("exists", lua.create_function( |_, path: String| {
         Ok(::std::path::Path::new(&path).exists())
     })?)?;