Use custom classifications like tags & categories

We used to classify posts with tags & categories.

This patch adds a generic way to define & use your own classifications through `site.config[:classifications]` by adding `tags` like behavior to site & posts for added classifications.

Let's say you want classify Post by referring to Project(s), Customer(s) and Layout(s) you can now simply add the following to `_config.yml` :

```
classifications: 
  - projects
  - customers
  - layouts
```

And start using those classifications like you would with `categories` or `tags` :

Add the following to a Post:

```
project: bartender
layout:
  - grid
  - responsive
```

And then use the following in any template :

```
<h2>Projects</h2>
<ul>
{% for project in site.projects %}
  <li>{{ project | first }}</li>
{% endfor %}
</ul>
```