Fix a synchronisation bug

The run method will almost always be called inside a thread. This commit fixes
a synchronisation problem between the run and stop methods by explicitly opening
a pipe on the beginning of the run method.

The source of the problem is the fact that when someone stops the watcher,
the @path would become nil and calling the private method pipe would create a new pipe.

A sitiuation where this could be a problem is when the run method is waiting on changes
(blocking on select), note here that @running does not matter anymore, the code is blocking
on select!

Stoping the watcher and firing an event at the same time would unblock the run method
and getting the results in the old code would have created a new pipe and blocked on readline.

lib/rb-fsevent/fsevent.rb

@@ -32,12 +32,14 @@ def watch(watch_paths, options=nil, &block)
   end
 
   def run
+    @pipe    = open_pipe
     @running = true
+
     # please note the use of IO::select() here, as it is used specifically to
     # preserve correct signal handling behavior in ruby 1.8.
-    while @running && IO::select([pipe], nil, nil, nil)
-      if line = pipe.readline
-        modified_dir_paths = line.split(":").select { |dir| dir != "\n" }
+    while @running && IO::select([@pipe], nil, nil, nil)
+      if line = @pipe.readline
+        modified_dir_paths = line.split(':').select { |dir| dir != "\n" }
         callback.call(modified_dir_paths)
       end
     end
@@ -47,18 +49,18 @@ def run
   end
 
   def stop
-    if pipe
-      Process.kill("KILL", pipe.pid)
-      pipe.close
+    unless @pipe.nil?
+      Process.kill('KILL', @pipe.pid)
+      @pipe.close
     end
   rescue IOError
   ensure
-    @pipe = @running = nil
+    @running = false
   end
 
   if RUBY_VERSION < '1.9'
-    def pipe
-      @pipe ||= IO.popen("#{self.class.watcher_path} #{options_string} #{shellescaped_paths}")
+    def open_pipe
+      IO.popen("#{self.class.watcher_path} #{options_string} #{shellescaped_paths}")
     end
 
     private
@@ -89,8 +91,8 @@ def shellescape(str)
       return str
     end
   else
-    def pipe
-      @pipe ||= IO.popen([self.class.watcher_path] + @options + @paths)
+    def open_pipe
+      IO.popen([self.class.watcher_path] + @options + @paths)
     end
   end