Updating README to fix precedence issue

Hello jeremy, 

Please review the following commits I made in branch 'vinny-readme-update'.

I was playing with the examples in the README to get a sense of 
Shepherd and I ran into the following issue:

```javascript
var graph = new require('shepherd').Graph
```

This would result in the following crash:

```
TypeError: Object function Graph() {
...
}
has no method 'add'
    at Object.<anonymous> (/Users/vinny/code/shepherd_test/test.js:6:7)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:901:3
```

This was reproduced in both node v0.8.20 and v0.10.5.

For this reason, I've updated to the following:

```javascript
var Graph = require('shepherd').Graph
  , graph = new Graph
```

This works fine.

R=jeremy