Improve pages collection url references.

When I was creating sitemap and navigation to my page, I faced plenty of issues with paths. Since I have the published site in ```build``` and the sourcecodes for it in ```src``` I was constantly doing hacks to get around the fact that all the path variables under a single page in ```pages``` collection contain these paths.

This a sample of a single page in ```pages``` collection:
```js
     { dirname: 'build/blog/first-blog-post',
       filename: 'index.html',
       pageName: 'index.html',
       pagename: 'index.html',
       basename: 'index',
       src: 'src/blog/first-blog-post/index.html.hbs',
       dest: 'build/blog/first-blog-post/index.html',
       assets: '../..',
       ext: '',
       extname: '',
       page: [Function],
       data: [Object],
       match: [Object] },
```

Now if I want to print out all the pages with links pointing to them, I have to remove the ```build/``` part from the path as that's where the root of my site is.

This pull request adds two new attributes ```url``` and ```url_stripped``` to the mix:
```js
     { dirname: 'build/blog/first-blog-post',
       filename: 'index.html',
       pageName: 'index.html',
       pagename: 'index.html',
       basename: 'index',
       src: 'src/blog/first-blog-post/index.html.hbs',
       dest: 'build/blog/first-blog-post/index.html',
       assets: '../..',
       ext: '',
       extname: '',
       page: [Function],
       data: [Object],
       url: 'blog/first-blog-post/index.html',
       url_stripped: 'blog/first-blog-post/',
       match: [Object] },
```
These two attributes only get added if the user has told assemble where the root of the site is in assemble.options:
```js
assemble: {
  options: {
    urlroot:'build/index.html',
...
```
The ```url``` attribute should be quite clear, it's the url to the page from the root of the site. The ```url_stripped``` is a handy way to clean up the *invisible* index.html files from paths. It also helps when sorting pages etc.

Here's a small example on listing the pages with this approach:
```html
<ul>
{{#withSort pages "url_stripped"}}
	<li>{{this.url_stripped}} - <a href="{{relative ../this.page.url this.url_stripped}}">{{this.data.title}}</a></li>
{{/withSort}}
</ul>
```
As you can see, it's quite easy to use and produces nice looking urls too.

Related to this, I found a sorting bug in the withSort and did a pull request for it together with my other pull request related to it.

Thoughts?