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`