Add `tick` hook and `daemon?` method.

This pull request has two parts:

`tick` callback
------------------
For integration with other systems other than Tmux, for example, integration with Mac OS X status bar, a `tick` hook is required.

```ruby
before do
  $ui = Gui.new
end

tick do |seconds_left|
  $ui.tick(seconds_left)
end

after do |seconds_left|
  $ui.destroy
end
```

`daemon?` method
-------------------------
Checking whether Thyme is in daemon mode is useful when you want to do some action that requires $stdin/$stdout access (which is only possible when not running in daemon mode). For example:

```ruby
after do |seconds_left|
  $ui.destroy
  if seconds_left == 0
    unless daemon?
      begin
        print "Start another Pomodoro?"
        gets
        exec "thyme"
      rescue Interrupt
        puts "Bye bye"
      end
    end
  end
end
```