disqus support + other things

## other things

- I updated sbt to 0.13 and gathered all build related things into `build.sbt`: eed3si9n/pamflet@08dc563d7e6fa406d6a2e00623505292f543a69b
- Then upgraded underlying libs to the latest: eed3si9n/pamflet@21998cf656f5159135c817778cbf4266202e7f4a
- Then I realized code highlighting is not working because script tag is using XML syntax, so I fixed it: eed3si9n/pamflet@40df0d7aef5ca1cca4d3b54303e415054404293c

## disqus support

This is pretty straight forward. Here's from the added docs:

> ### Disqus
>
> Defining `disqus` property with a disqus short name will add a comment section at the end of all your pages.
> 
>     disqus=namehere
>