The Porttime beta release

Since I hacked Porttime support into Portmidi for my RubyOnIce talk, I wanted to get it out there as quickly as possible, hence the beta release.

• Porttime.start gives you now access to the Porttime timer system. It suprisingly "just" works, but I'm pretty sure I've added some interesting bugs with this, as the timer system runs in a different thread.
• Portmidi.write_short now accepts a timestamp, but please be aware that these timestamps are only used to sort messages, not to delay their sending.