fibrous.run - a convenience function to quickly execute something in a fiber

Comes with its own tests and README update.

```javascript
fibrous.run(function() {
    var data = fs.sync.readFile('/etc/passwd');
    console.log(data.toString());
}, function(err) {
    console.log("Handle both async and sync errors here", err);
});
```

You can handle any errors inside the passed function with try/catch or 
by passing a second function that acts as an error handler.

When an error happens, if you don't use try/catch inside the first function and don't pass an error handler, you get an uncaught exception. This is similar to the way node event emitters act if you don't attach an error event handler.