Support UTF-8 strings

Fix bug for utf-8.

```ruby
# -*- encoding: utf-8 -*-
require 'ruby-growl'
require 'ruby-growl/ruby_logo'

g = Growl.new "localhost", "ruby-growl"
g.add_notification("notification", "ruby-growl Notification",
                   Growl::RUBY_LOGO_PNG)
g.notify "notification", "It came from ruby-growl", "UTF-8の文字列を送信"
```

    /Users/kunigaku/src/ruby-growl/lib/ruby-growl/gntp.rb:336:in `join': incompatible character encodings: UTF-8 and ASCII-8BIT (Encoding::CompatibilityError)
    	from /Users/kunigaku/src/ruby-growl/lib/ruby-growl/gntp.rb:336:in `packet'
    	from /Users/kunigaku/src/ruby-growl/lib/ruby-growl/gntp.rb:376:in `packet_notify'
    	from /Users/kunigaku/src/ruby-growl/lib/ruby-growl/gntp.rb:269:in `notify'
    	from /Users/kunigaku/src/ruby-growl/lib/ruby-growl.rb:305:in `notify_gntp'
    	from /Users/kunigaku/src/ruby-growl/lib/ruby-growl.rb:285:in `notify'
    	from rg.rb:8:in `<main>'