Add support for streaming to Syslog

I've kind of wanted this for a while but never really had a need. Inside GitHub we use rails/unicorn/god everywhere. Getting those to work together well, especially in our current transition to Scrolls logging, is quite painful.

Basically, if you are using Ruby 2.0.0, you can specify:

```ruby
Scrolls.stream = 'syslog'
```

Scrolls will send messages to `user.info` in Syslog.

I plan to extend this to allow one to specify facility or level.

/cc @imbriaco