Fix mod usage (#72)

* Use the proper path for FluentBundle
* Drop the `extern crate...` ceremony for 2018 edition

Author: JohnHeitmann

fluent/README.md

@@ -22,9 +22,7 @@ Usage
 -----
 
 ```rust
-extern crate fluent;
-
-use fluent::FluentBundle;
+use fluent::bundle::FluentBundle;
 
 fn main() {
     let mut bundle = FluentBundle::new(&["en-US"]);