Add twitter timeline widget


I've modified the templates to add a Twitter Timeline Widget in the sidebar. 

Instructions for set up:
In the `pelicanconf.py` include the following variables:
`TWITTER_WIDGET_ID` = XXXXX
`TWITTER_USERNAME` = XXXXX

To get a `TWITTER_WIDGET_ID`, go to: https://twitter.com/settings/widgets and `Create new`. You'll find the `TWITTER_WIDGET_ID` under the html or in the site url:

`https://twitter.com/settings/widgets/TWITTER_WIDGET_ID/edit`

Here's a Snapshot of how it looks in your readable-old theme. I've added it under the Github Activity:

![twitter-timeline-pr](https://f.cloud.github.com/assets/4940549/2034237/fa6afa6e-8927-11e3-9b39-efbb880b8fa3.png)