Update rocket_i18n and add gettext_macros (#431)

Internationalization now uses proc-macros that generate the .pot file
automatically.

build.rs

@@ -1,5 +1,4 @@
 extern crate ructe;
-extern crate rocket_i18n;
 extern crate rsass;
 use ructe::*;
 use std::{env, fs::*, io::Write, path::PathBuf};
@@ -10,10 +9,6 @@ fn main() {
         .join("templates");
     compile_templates(&in_dir, &out_dir).expect("compile templates");
 
-    println!("cargo:rerun-if-changed=po");
-    rocket_i18n::update_po("plume", &["de", "en", "fr", "gl", "it", "ja", "nb", "pl", "ru"]);
-    rocket_i18n::compile_po("plume", &["de", "en", "fr", "gl", "it", "ja", "nb", "pl", "ru"]);
-
     println!("cargo:rerun-if-changed=static/css");
     let mut out = File::create("static/css/main.css").expect("Couldn't create main.css");
     out.write_all(