Feature: category #317

This is the implementation of #317
Here are some to-dos for me:
- [x] Deploy a live site to test
- [x] UI needs to be changed 
- [x] **Color** of the UI not implemented

@elkuku @b2z Please review the code. And especially the `etc/mysql.sql` file. I will probably deploy a live site later today, but need your advices on how(or where, etc), so you can test with full - feature. Do we need a fake account and fake repo?

And for the UI part, any ideas?

Thanks!
May be instead of showing `No categories...` it is better to hide all `ul`? Like:
```html
{% if item.categories %}
    <ul class="breadcrumb">
```

And this won't be needed:
```html
{% else %}
    <li>
        {{ 'No categories choosen.'|_ }}
    </li>
```