Allow for graceful failure if the server is not running

Hey @chrismccord --

Wondering if you think this change would be useful? In a couple of my projects that use sync, I don't need to run Faye during development, and with this change, it no longer chokes when the server is not running.

Thanks for all your work on this great library!
It's not DRY, but I liked it better than:

```coffee
return unless (adapter = Sync[@CLIENT_ADAPTER])
@adapter = new adapter
```