Introduce template for navigation

Hi guys,

I was thinking to make the navigation customizable via template instead of hard-coded.
Important are the classes that must be set: `popover-footer` (the container of the navigation), `prev`, `next`, `separator`, `end`.

E.g.:
````html
<div class='popover tour'>
  <div class='arrow'></div>
  <h3 class='popover-title'></h3>
  <div class='popover-content'></div>
  <div class='popover-footer'>
    <a class='prev'></a>
    <span class='separator'></span>
    <a class='next'></a>
    <a class='end'></a>
  </div>
</div>
```
What do you think?