Feature/delete expired

This adds a purge option to zerobin (https://github.com/sametmax/0bin/issues/48) by adding two class method to the Paste class (see [paste.py#L296](https://github.com/jcsaaddupuy/0bin/blob/feature/delete_expired/zerobin/paste.py#L296) and [paste.py#L315](https://github.com/jcsaaddupuy/0bin/blob/feature/delete_expired/zerobin/paste.py#L315))

I add to move some logic from the route to the Paste class (see [paste.py#L268](https://github.com/jcsaaddupuy/0bin/blob/feature/delete_expired/zerobin/paste.py#L268))

I also added a is_burn_notice property (only for aestheticism)

With this patch, zerobin can be called this way:
```bash
zerobin --purge
```

It could also be done in a route like /purge if needed.

(and it seems that I haven't broken anything :) )