Added script to list last 3 twitter mentions

Lists mentions for whichever account owns the API credentials. Twitter in pure JavaScript because CoffeeScript was making me insane.

## Future Features

Here's a list of things I plan to do in the near future but I'd rather get something working out now.

* More configuration options for number of recent tweets.
* Interval so hubot can automatically alert us of incoming tweets.
* Support for multiple twitter accounts (MetaMesh and PittMesh).

## Configuration

Set the following environment variables. These keys should belong to https://twitter.com/metameshllc.

* TWITTER_CONSUMER_KEY
* TWITTER_CONSUMER_SECRET
* TWITTER_ACCESS_TOKEN
* TWITTER_ACCESS_TOKEN_SECRET

## Usage

Pretty simple.

`hubot show twitter mentions`