Added Calendar next/pre month links

fixes #726 
SS below - 
![calinks](https://cloud.githubusercontent.com/assets/1424236/4935484/78d712ae-65ad-11e4-99cc-dceb75400329.png)

You're also going to want to rework the query to include years.  I jumped[0] from the month of my birth to July 2015:

```perl
my $prev_query = uri_esc('calendar '.$previous->strftime("%B %Y"));
```

Or similar, whatever makes sense to you.

[0] Yes, this means I got it working under duckpan. :grin: