Fix broken file watcher implementation

This one took a little bit of tracking down; the VfsWatcher used to spawn a new
thread and then stall/park forever. With one of the recent changes to get rid of
the extra thread, VfsWatcher started getting dropped, which in turn dropped the
watchers created by the notify crate.

Because the threads only tie back to the VfsWatcher was a cloned
Arc<Mutex<VfsSession>>, everything was fine, except that their mpsc::Receiver
objects were no longer receiving events.

This manifested itself as the file watcher magically not watching any files.
Oops.

src/commands/serve.rs

@@ -1,6 +1,7 @@
 use std::path::{Path, PathBuf};
-use std::sync::{Arc, Mutex};
 use std::process;
+use std::sync::{Arc, Mutex};
+use std::thread;
 
 use rand;
 
@@ -87,6 +88,12 @@ pub fn serve(project_path: &PathBuf, verbose: bool, port: Option<u64>) {
 
     println!("Server listening on port {}", web_config.port);
 
-    VfsWatcher::new(vfs.clone()).start();
+    {
+        let vfs = vfs.clone();
+        thread::spawn(move || {
+            VfsWatcher::new(vfs).start();
+        });
+    }
+
     web::start(web_config, project.clone(), &PLUGIN_CHAIN, vfs.clone());
 }