Adding prefix/suffix options to twitter plugin config

Based on @woodrowbarlow's PR #207 (now closed) - I have taken those changes and cleaned them up to provide a cleaner approach for configuring a prefix and a suffix for tweets sent from our twitter plugin. To summarise;

> The user will be prompted to specify an optional prefix or suffix (such as an `@user` and custom `#hashtag`) when enabling the twitter plugin. That information will be stored in the config.yml file.  If no suffix is supplied, a default hash tag, `#lolcommits` will always be added at the end of each tweet.

A new test was added to cover building tweets in this way.  I have also updated the twitter and json gems and set a specific gem version for oath `0.4.7`.  This was necessary to fix some oauth/token issues with the latest Twitter API.

Thanks to @woodrowbarlow for your initial contribution!

This PR closes #161 and will be released soon!