Make it so that handler functions default to Graph#subgraph if left unspecified

Hello @nicks, 

Please review the following commits I made in branch 'sho-syntax'.

This is to allow the following syntax:

```js
// New syntax
graph.add('square')
  .args('x')
  .builds('multiply').using({x: 'args.x', y: 'args.x'})

// Old syntax
graph.add('sixteen', graph.subgraph, ['x'])
  .builds('square').using({x: 4})
```

7ad3e6bc6a40e5694bd7ef3af276c0ae8b132f17 (2013-08-28 11:24:55 -0700)
Make it so that handler functions default to Graph#subgraph if left unspecified


R=@nicks