Add stem.util.conf.Config.save()

Added a new save method that saves the current config contents into the configuration file by overwriting the old one.

  def save(self):
    self._contents_lock.acquire()

    with open(path, 'w') as f:
      for entry in self.keys():
        f.write('%s %s\n' % (entry, self.get(entry)))

    self._contents_lock.release()

If we want to retain the comments --

  def save(self):
    self._contents_lock.acquire()

    with open(path, 'w') as f:
      for line in self._raw_contents:
        key, value = ""
        comment_start = line.find("#")
        if comment_start != -1:
          comment = line[comment_start:]
          line = line[:comment_start]
        line = line.strip()
      
        if line:
          try:
            key, value = line.split(" ", 1)
          except ValueError:
            key, value = line, ""
    
        f.write("%s %s %s\n" % (key, self.get(key, value), comment)
      
    self._contents_lock.release()

Since we strip the lines, we don't really know what the formatting is like. This messes up the white spaces a bit.

Do we have an alternative? Or is one these fine?