Other exports from casper module

`casper` module provides an XPath selector helper called `selectXPath` in `casperjs`. 

I think it would be handy to expose `selectXPath` in `cli.js` like thus:

```js
// Load xPath helper
this.selectXPath = require('casper').selectXPath;

// Load casper
this.casper = require('casper').create({
...
```

Are you cool with that - or do you have other ideas around structuring the `this` instance? Shall I make a PR?