Improved pages array to include relative positions

This pull requests is a proposal to add relative positions in the page array.
It makes graphical components easier to build, like in Twig views.

The relative positions are:
* **first**: Unique, the first page
* **less**: Unique, was previously "..."
* **previous**: One or more, all previous pages in the neighbours
* **current**: Unique, the current page
* **next**: One or more, all next pages in the neighbours
* **more**: Unique, was previously "..."
* **last**: Unique, the latest page


The new generated array is:
```php
array(
        1 => 'first',
        5 => 'less',
        6 => 'previous',
        7 => 'previous',
        8 => 'previous',
        9 => 'previous',
        10 => 'current',
        11 => 'next',
        12 => 'next',
        13 => 'next',
        14 => 'next',
        15 => 'more',
        20 => 'last'
    )
```

Thus, we can build graphical components in Twig very easily, like this one:
```
<ul>
   {% for number, position in pages %}
     {% if position == 'first' %}
       <li class="button action">&laquo;</li>
     {% elseif position == 'previous' or position == 'next' %}
       <li class="button">{{ number }}</li>
     {% elseif position == 'current' %}
       <li class="button active">{{ number }}</li>
     {% elseif position == 'last' %}
       <li class="button action">&raquo;</li>
     {% endif %}
   {% endfor %}
 </ul>
```

The ```README``` and tests have been updated.

Any comment or suggestion are welcome, alright :wink: